Ada95 Reference Manual

Table of Contents

Foreword

  1. ISO (the International Organization for Standardization) and IEC (the International Electrotechnical Commission) form the specialized system for worldwide standardization. National bodies that are members of ISO or IEC participate in the development of International Standards through technical committees established by the respective organization to deal with particular fields of technical activity. ISO and IEC technical committees collaborate in fields of mutual interest. Other international organizations, governmental and non-governmental, in liaison with ISO and IEC, also take part in the work.
  2. In the field of information technology, ISO and IEC have established a joint technical committee, ISO/IEC JTC 1. Draft International Standards adopted by the joint technical committee are circulated to national bodies for voting. Publication as an International Standard requires approval by at least 75 % of the national bodies casting a vote.
  3. International Standard ISO/IEC 8652 was prepared by Joint Technical Committee ISO/IEC JTC 1, Information Technology.
  4. This second edition cancels and replaces the first edition (ISO 8652:1987), of which it constitutes a technical revision.
  5. Annexes A to J form an integral part of this International Standard. Annexes K to P are for information only.

Introduction

  1. This is the Ada Reference Manual.
  2. Other available Ada documents include:
    1. Rationale for the Ada Programming Language -- 1995 edition, which gives an introduction to the new features of Ada, and explains the rationale behind them. Programmers should read this first.
    2. Changes to Ada -- 1987 to 1995. This document lists in detail the changes made to the 1987 edition of the standard.
    3. The Annotated Ada Reference Manual (AARM). The AARM contains all of the text in the RM95, plus various annotations. It is intended primarily for compiler writers, validation test writers, and others who wish to study the fine details. The annotations include detailed rationale for individual rules and explanations of some of the more arcane interactions among the rules.

Design Goals

  1. Ada was originally designed with three overriding concerns: program reliability and maintenance, programming as a human activity, and efficiency. This revision to the language was designed to provide greater flexibility and extensibility, additional control over storage management and synchronization, and standardized packages oriented toward supporting important application areas, while at the same time retaining the original emphasis on reliability, maintainability, and efficiency.
  2. The need for languages that promote reliability and simplify maintenance is well established. Hence emphasis was placed on program readability over ease of writing. For example, the rules of the language require that program variables be explicitly declared and that their type be specified. Since the type of a variable is invariant, compilers can ensure that operations on variables are compatible with the properties intended for objects of the type. Furthermore, error-prone notations have been avoided, and the syntax of the language avoids the use of encoded forms in favor of more English-like constructs. Finally, the language offers support for separate compilation of program units in a way that facilitates program development and maintenance, and which provides the same degree of checking between units as within a unit.
  3. Concern for the human programmer was also stressed during the design. Above all, an attempt was made to keep to a relatively small number of underlying concepts integrated in a consistent and systematic way while continuing to avoid the pitfalls of excessive involution. The design especially aims to provide language constructs that correspond intuitively to the normal expectations of users.
  4. Like many other human activities, the development of programs is becoming ever more decentralized and distributed. Consequently, the ability to assemble a program from independently produced software components continues to be a central idea in the design. The concepts of packages, of private types, and of generic units are directly related to this idea, which has ramifications in many other aspects of the language. An allied concern is the maintenance of programs to match changing requirements; type extension and the hierarchical library enable a program to be modified while minimizing disturbance to existing tested and trusted components.
  5. No language can avoid the problem of efficiency. Languages that require over-elaborate compilers, or that lead to the inefficient use of storage or execution time, force these inefficiencies on all machines and on all programs. Every construct of the language was examined in the light of present implementation techniques. Any proposed construct whose implementation was unclear or that required excessive machine resources was rejected.

Language Summary

  1. An Ada program is composed of one or more program units. Program units may be subprograms (which define executable algorithms), packages (which define collections of entities), task units (which define concurrent computations), protected units (which define operations for the coordinated sharing of data between tasks), or generic units (which define parameterized forms of packages and subprograms). Each program unit normally consists of two parts: a specification, containing the information that must be visible to other units, and a body, containing the implementation details, which need not be visible to other units. Most program units can be compiled separately.
  2. This distinction of the specification and body, and the ability to compile units separately, allows a program to be designed, written, and tested as a set of largely independent software components.
  3. An Ada program will normally make use of a library of program units of general utility. The language provides means whereby individual organizations can construct their own libraries. All libraries are structured in a hierarchical manner; this enables the logical decomposition of a subsystem into individual components. The text of a separately compiled program unit must name the library units it requires.

Program Units

  1. A subprogram is the basic unit for expressing an algorithm. There are two kinds of subprograms: procedures and functions. A procedure is the means of invoking a series of actions. For example, it may read data, update variables, or produce some output. It may have parameters, to provide a controlled means of passing information between the procedure and the point of call. A function is the means of invoking the computation of a value. It is similar to a procedure, but in addition will return a result.
  2. A package is the basic unit for defining a collection of logically related entities. For example, a package can be used to define a set of type declarations and associated operations. Portions of a package can be hidden from the user, thus allowing access only to the logical properties expressed by the package specification.
  3. Subprogram and package units may be compiled separately and arranged in hierarchies of parent and child units giving fine control over visibility of the logical properties and their detailed implementation.
  4. A task unit is the basic unit for defining a task whose sequence of actions may be executed concurrently with those of other tasks. Such tasks may be implemented on multicomputers, multiprocessors, or with interleaved execution on a single processor. A task unit may define either a single executing task or a task type permitting the creation of any number of similar tasks.
  5. A protected unit is the basic unit for defining protected operations for the coordinated use of data shared between tasks. Simple mutual exclusion is provided automatically, and more elaborate sharing protocols can be defined. A protected operation can either be a subprogram or an entry. A protected entry specifies a Boolean expression (an entry barrier) that must be true before the body of the entry is executed. A protected unit may define a single protected object or a protected type permitting the creation of several similar objects.

Declarations and Statements

  1. The body of a program unit generally contains two parts: a declarative part, which defines the logical entities to be used in the program unit, and a sequence of statements, which defines the execution of the program unit.
  2. The declarative part associates names with declared entities. For example, a name may denote a type, a constant, a variable, or an exception. A declarative part also introduces the names and parameters of other nested subprograms, packages, task units, protected units, and generic units to be used in the program unit.
  3. The sequence of statements describes a sequence of actions that are to be performed. The statements are executed in succession (unless a transfer of control causes execution to continue from another place).
  4. An assignment statement changes the value of a variable. A procedure call invokes execution of a procedure after associating any actual parameters provided at the call with the corresponding formal parameters.
  5. Case statements and if statements allow the selection of an enclosed sequence of statements based on the value of an expression or on the value of a condition.
  6. The loop statement provides the basic iterative mechanism in the language. A loop statement specifies that a sequence of statements is to be executed repeatedly as directed by an iteration scheme, or until an exit statement is encountered.
  7. A block statement comprises a sequence of statements preceded by the declaration of local entities used by the statements.
  8. Certain statements are associated with concurrent execution. A delay statement delays the execution of a task for a specified duration or until a specified time. An entry call statement is written as a procedure call statement; it requests an operation on a task or on a protected object, blocking the caller until the operation can be performed. A called task may accept an entry call by executing a corresponding accept statement, which specifies the actions then to be performed as part of the rendezvous with the calling task. An entry call on a protected object is processed when the corresponding entry barrier evaluates to true, whereupon the body of the entry is executed. The requeue statement permits the provision of a service as a number of related activities with preference control. One form of the select statement allows a selective wait for one of several alternative rendezvous. Other forms of the select statement allow conditional or timed entry calls and the asynchronous transfer of control in response to some triggering event.
  9. Execution of a program unit may encounter error situations in which normal program execution cannot continue. For example, an arithmetic computation may exceed the maximum allowed value of a number, or an attempt may be made to access an array component by using an incorrect index value. To deal with such error situations, the statements of a program unit can be textually followed by exception handlers that specify the actions to be taken when the error situation arises. Exceptions can be raised explicitly by a raise statement.

Data Types

  1. Every object in the language has a type, which characterizes a set of values and a set of applicable operations. The main classes of types are elementary types (comprising enumeration, numeric, and access types) and composite types (including array and record types).
  2. An enumeration type defines an ordered set of distinct enumeration literals, for example a list of states or an alphabet of characters. The enumeration types Boolean, Character, and Wide_Character are predefined.
  3. Numeric types provide a means of performing exact or approximate numerical computations. Exact computations use integer types, which denote sets of consecutive integers. Approximate computations use either fixed point types, with absolute bounds on the error, or floating point types, with relative bounds on the error. The numeric types Integer, Float, and Duration are predefined.
  4. Composite types allow definitions of structured objects with related components. The composite types in the language include arrays and records. An array is an object with indexed components of the same type. A record is an object with named components of possibly different types. Task and protected types are also forms of composite types. The array types String and Wide_String are predefined.
  5. Record, task, and protected types may have special components called discriminants which parameterize the type. Variant record structures that depend on the values of discriminants can be defined within a record type.
  6. Access types allow the construction of linked data structures. A value of an access type represents a reference to an object declared as aliased or to an object created by the evaluation of an allocator. Several variables of an access type may designate the same object, and components of one object may designate the same or other objects. Both the elements in such linked data structures and their relation to other elements can be altered during program execution. Access types also permit references to subprograms to be stored, passed as parameters, and ultimately dereferenced as part of an indirect call.
  7. Private types permit restricted views of a type. A private type can be defined in a package so that only the logically necessary properties are made visible to the users of the type. The full structural details that are externally irrelevant are then only available within the package and any child units.
  8. From any type a new type may be defined by derivation. A type, together with its derivatives (both direct and indirect) form a derivation class. Class-wide operations may be defined that accept as a parameter an operand of any type in a derivation class. For record and private types, the derivatives may be extensions of the parent type. Types that support these object-oriented capabilities of class-wide operations and type extension must be tagged, so that the specific type of an operand within a derivation class can be identified at run time. When an operation of a tagged type is applied to an operand whose specific type is not known until run time, implicit dispatching is performed based on the tag of the operand.
  9. The concept of a type is further refined by the concept of a subtype, whereby a user can constrain the set of allowed values of a type. Subtypes can be used to define subranges of scalar types, arrays with a limited set of index values, and records and private types with particular discriminant values.

Other Facilities

  1. Representation clauses can be used to specify the mapping between types and features of an underlying machine. For example, the user can specify that objects of a given type must be represented with a given number of bits, or that the components of a record are to be represented using a given storage layout. Other features allow the controlled use of low level, nonportable, or implementation-dependent aspects, including the direct insertion of machine code.
  2. The predefined environment of the language provides for input-output and other capabilities (such as string manipulation and random number generation) by means of standard library packages. Input-output is supported for values of user-defined as well as of predefined types. Standard means of representing values in display form are also provided. Other standard library packages are defined in annexes of the standard to support systems with specialized requirements.
  3. Finally, the language provides a powerful means of parameterization of program units, called generic program units. The generic parameters can be types and subprograms (as well as objects and packages) and so allow general algorithms and data structures to be defined that are applicable to all types of a given class.

Language Changes

  1. This International Standard replaces the first edition of 1987. In this edition, the following major language changes have been incorporated:
    1. Support for standard 8-bit and 16-bit character sets. See section Lexical Elements, section Character Types, section String Types, section The Package Standard, section Character Handling, and section String Handling.
    2. Object-oriented programming with run-time polymorphism. See the discussions of classes, derived types, tagged types, record extensions, and private extensions in clauses section Derived Types and Classes, section Tagged Types and Type Extensions, and section Private Types and Private Extensions. See also the new forms of generic formal parameters that are allowed by section Formal Private and Derived Types, and section Formal Packages.
    3. Access types have been extended to allow an access value to designate a subprogram or an object declared by an object declaration (as opposed to just a heap-allocated object) (see section Access Types).
    4. Efficient data-oriented synchronization is provided via protected types. See section Tasks and Synchronization.
    5. The library units of a library may be organized into a hierarchy of parent and child units (see section Program Structure and Compilation Issues).
    6. Additional support has been added for interfacing to other languages. See section Interface to Other Languages (normative).
    7. The Specialized Needs Annexes have been added to provide specific support for certain application areas:
      1. Annex C, "Systems Programming"
      2. Annex D, "Real-Time Systems"
      3. Annex E, "Distributed Systems"
      4. Annex F, "Information Systems"
      5. Annex G, "Numerics"
      6. Annex H, "Safety and Security"

Instructions for Comment Submission

  1. Informal comments on this International Standard may be sent via e-mail to ada-comment@sw-eng.falls-church.va.us. If appropriate, the Project Editor will initiate the defect correction procedure.
  2. Comments should use the following format: 
  3. !topic Title summarizing comment
!reference RM95-ss.ss(pp)
!from Author Name yy-mm-dd
!keywords keywords related to topic
!discussion

text of discussion
  4. where ss.ss is the section, clause or subclause number, pp is the paragraph number where applicable, and yy-mm-dd is the date the comment was sent. The date is optional, as is the !keywords line.
  5. Multiple comments per e-mail message are acceptable. Please use a descriptive "Subject" in your e-mail message.
  6. When correcting typographical errors or making minor wording suggestions, please put the correction directly as the topic of the comment; use square brackets [ ] to indicate text to be omitted and curly braces { } to indicate text to be added, and provide enough context to make the nature of the suggestion self-evident or put additional information in the body of the comment, for example: 
  7. !topic [c]{C}haracter
!topic it[']s meaning is not defined
  8. Formal requests for interpretations and for reporting defects in this International Standard may be made in accordance with the ISO/IEC JTC1 Directives and the ISO/IEC JTC1/SC22 policy for interpretations. National Bodies may submit a Defect Report to ISO/IEC JTC1/SC22 for resolution under the JTC1 procedures. A response will be provided and, if appropriate, a Technical Corrigendum will be issued in accordance with the procedures.

Acknowledgements

  1. This International Standard was prepared by the Ada 9X Mapping/Revision Team based at Intermetrics, Inc., which has included: W. Carlson, Program Manager; T. Taft, Technical Director; J. Barnes (consultant); B. Brosgol (consultant); R. Duff (Oak Tree Software); M. Edwards; C. Garrity; R. Hilliard; O. Pazy (consultant); D. Rosenfeld; L. Shafer; W. White; M. Woodger.
  2. The following consultants to the Ada 9X Project contributed to the Specialized Needs Annexes: T. Baker (Real-Time/Systems Programming -- SEI, FSU); K. Dritz (Numerics -- Argonne National Laboratory); A. Gargaro (Distributed Systems -- Computer Sciences); J. Goodenough (Real-Time/Systems Programming -- SEI); J. McHugh (Secure Systems -- consultant); B. Wichmann (Safety-Critical Systems -- NPL: UK).
  3. This work was regularly reviewed by the Ada 9X Distinguished Reviewers and the members of the Ada 9X Rapporteur Group (XRG): E. Ploedereder, Chairman of DRs and XRG (University of Stuttgart: Germany); B. Bardin (Hughes); J. Barnes (consultant: UK); B. Brett (DEC); B. Brosgol (consultant); R. Brukardt (RR Software); N. Cohen (IBM); R. Dewar (NYU); G. Dismukes (TeleSoft); A. Evans (consultant); A. Gargaro (Computer Sciences); M. Gerhardt (ESL); J. Goodenough (SEI); S. Heilbrunner (University of Salzburg: Austria); P. Hilfinger (UC/Berkeley); B. Kaellberg (CelsiusTech: Sweden); M. Kamrad II (Unisys); J. van Katwijk (Delft University of Technology: The Netherlands); V. Kaufman (Russia); P. Kruchten (Rational); R. Landwehr (CCI: Germany); C. Lester (Portsmouth Polytechnic: UK); L. Mansson (TELIA Research: Sweden); S. Michell (Multiprocessor Toolsmiths: Canada); M. Mills (US Air Force); D. Pogge (US Navy); K. Power (Boeing); O. Roubine (Verdix: France); A. Strohmeier (Swiss Fed Inst of Technology: Switzerland); W. Taylor (consultant: UK); J. Tokar (Tartan); E. Vasilescu (Grumman); J. Vladik (Prospeks s.r.o.: Czech Republic); S. Van Vlierberghe (OFFIS: Belgium).
  4. Other valuable feedback influencing the revision process was provided by the Ada 9X Language Precision Team (Odyssey Research Associates), the Ada 9X User/Implementer Teams (AETECH, Tartan, TeleSoft), the Ada 9X Implementation Analysis Team (New York University) and the Ada community-at-large.
  5. Special thanks go to R. Mathis, Convenor of ISO/IEC JTC1/SC22 Working Group 9.
  6. The Ada 9X Project was sponsored by the Ada Joint Program Office. Christine M. Anderson at the Air Force Phillips Laboratory (Kirtland AFB, NM) was the project manager. Changes
  7. The International Standard is the same as this version of the Reference Manual, except:
    1. This list of Changes is not included in the International Standard.
    2. The "Acknowledgements" page is not included in the International Standard.
    3. The text in the running headers and footers on each page is slightly different in the International Standard.
    4. The title page(s) are different in the International Standard.
    5. This document is formatted for 8.5-by-11-inch paper, whereas the International Standard is formatted for A4 paper (210-by-297mm); thus, the page breaks are in different places.

General

  1. Ada is a programming language designed to support the construction of long-lived, highly reliable software systems. The language includes facilities to define packages of related types, objects, and operations. The packages may be parameterized and the types may be extended to support the construction of libraries of reusable, adaptable software components. The operations may be implemented as subprograms using conventional sequential control structures, or as entries that include synchronization of concurrent threads of control as part of their invocation. The language treats modularity in the physical sense as well, with a facility to support separate compilation.
  2. The language includes a complete facility for the support of real-time, concurrent programming. Errors can be signaled as exceptions and handled explicitly. The language also covers systems programming; this requires precise control over the representation of data and access to system-dependent properties. Finally, a predefined environment of standard packages is provided, including facilities for, among others, input-output, string manipulation, numeric elementary functions, and random number generation.

Scope

  1. This International Standard specifies the form and meaning of programs written in Ada. Its purpose is to promote the portability of Ada programs to a variety of data processing systems.

Extent

  1. This International Standard specifies:
    1. The form of a program written in Ada;
    2. The effect of translating and executing such a program;
    3. The manner in which program units may be combined to form Ada programs;
    4. The language-defined library units that a conforming implementation is required to supply;
    5. The permissible variations within the standard, and the manner in which they are to be documented;
    6. Those violations of the standard that a conforming implementation is required to detect, and the effect of attempting to translate or execute a program containing such violations;
    7. Those violations of the standard that a conforming implementation is not required to detect.
  1. This International Standard does not specify:
    1. The means whereby a program written in Ada is transformed into object code executable by a processor;
    2. The means whereby translation or execution of programs is invoked and the executing units are controlled;
    3. The size or speed of the object code, or the relative execution speed of different language constructs; The form or contents of any listings produced by implementations; in particular, the form or contents of error or warning messages;
    4. The effect of unspecified execution.
    5. The size of a program or program unit that will exceed the capacity of a particular conforming implementation.

Structure

  1. This International Standard contains thirteen sections, fourteen annexes, and an index.
  2. The core of the Ada language consists of:
    1. Sections 1 through 13
    2. Annex A, "Predefined Language Environment"
    3. Annex B, "Interface to Other Languages"
    4. Annex J, "Obsolescent Features"
  1. The following Specialized Needs Annexes define features that are needed by certain application areas:
    1. Annex C, "Systems Programming"
    2. Annex D, "Real-Time Systems"
    3. Annex E, "Distributed Systems"
    4. Annex F, "Information Systems"
    5. Annex G, "Numerics"
    6. Annex H, "Safety and Security"
  1. The core language and the Specialized Needs Annexes are normative, except that the material in each of the items listed below is informative:
    1. Text under a NOTES or Examples heading.
    2. Each clause or subclause whose title starts with the word "Example" or "Examples".
  1. All implementations shall conform to the core language. In addition, an implementation may conform separately to one or more Specialized Needs Annexes.
  2. The following Annexes are informative:
    1. Annex K, "Language-Defined Attributes"
    2. Annex L, "Language-Defined Pragmas"
    3. Annex M, "Implementation-Defined Characteristics"
    4. Annex N, "Glossary"
    5. Annex P, "Syntax Summary"
  1. Each section is divided into clauses and subclauses that have a common structure. Each section, clause, and subclause first introduces its subject. After the introductory text, text is labeled with the following headings:

    Syntax

  2. Syntax rules (indented).

    Name Resolution Rules

  3. Compile-time rules that are used in name resolution, including overload resolution.

    Legality Rules

  4. Rules that are enforced at compile time. A construct is legal if it obeys all of the Legality Rules.

    Static Semantics

  5. A definition of the compile-time effect of each construct.

    Post-Compilation Rules

  6. Rules that are enforced before running a partition. A partition is legal if its compilation units are legal and it obeys all of the Post-Compilation Rules.

    Dynamic Semantics

  7. A definition of the run-time effect of each construct.

    Bounded (Run-Time) Errors

  8. Situations that result in bounded (run-time) errors, see section Classification of Errors.

    Erroneous Execution

  9. Situations that result in erroneous execution, see section Classification of Errors.

    Implementation Requirements

  10. Additional requirements for conforming implementations.

    Documentation Requirements

  11. Documentation requirements for conforming implementations.

    Metrics

  12. Metrics that are specified for the time/space properties of the execution of certain language constructs.

    Implementation Permissions

  13. Additional permissions given to the implementer.

    Implementation Advice

  14. Optional advice given to the implementer. The word "should" is used to indicate that the advice is a recommendation, not a requirement. It is implementation defined whether or not a given recommendation is obeyed. NOTES
  15. (1) Notes emphasize consequences of the rules described in the (sub)clause or elsewhere. This material is informative.

    Examples

  16. Examples illustrate the possible forms of the constructs described. This material is informative.

Conformity of an Implementation with the Standard

Implementation Requirements

  1. A conforming implementation shall:
    1. Translate and correctly execute legal programs written in Ada, provided that they are not so large as to exceed the capacity of the implementation;
    2. Identify all programs or program units that are so large as to exceed the capacity of the implementation (or raise an appropriate exception at run time);
    3. Identify all programs or program units that contain errors whose detection is required by this International Standard;
    4. Supply all language-defined library units required by this International Standard;
    5. Contain no variations except those explicitly permitted by this International Standard, or those that are impossible or impractical to avoid given the implementation's execution environment;
    6. Specify all such variations in the manner prescribed by this International Standard.
  1. The external effect of the execution of an Ada program is defined in terms of its interactions with its external environment. The following are defined as external interactions:
    1. Any interaction with an external file, see section External Files and File Objects,
    2. The execution of certain code_statements, see section Machine Code Insertions, which code_statements cause external interactions is implementation defined.
    3. Any call on an imported subprogram, see section Interface to Other Languages (normative), including any parameters passed to it;
    4. Any result returned or exception propagated from a main subprogram (see section Program Execution) or an exported subprogram, see section Interface to Other Languages (normative), to an external caller;
    5. Any read or update of an atomic or volatile object, see section Shared Variable Control,
    6. The values of imported and exported objects, see section Interface to Other Languages (normative), at the time of any other interaction with the external environment.
  1. A conforming implementation of this International Standard shall produce for the execution of a given Ada program a set of interactions with the external environment whose order and timing are consistent with the definitions and requirements of this International Standard for the semantics of the given program.
  2. An implementation that conforms to this Standard shall support each capability required by the core language as specified. In addition, an implementation that conforms to this Standard may conform to one or more Specialized Needs Annexes (or to none). Conformance to a Specialized Needs Annex means that each capability required by the Annex is provided as specified.
  3. An implementation conforming to this International Standard may provide additional attributes, library units, and pragmas. However, it shall not provide any attribute, library unit, or pragma having the same name as an attribute, library unit, or pragma (respectively) specified in a Specialized Needs Annex unless the provided construct is either as specified in the Specialized Needs Annex or is more limited in capability than that required by the Annex. A program that attempts to use an unsupported capability of an Annex shall either be identified by the implementation before run time or shall raise an exception at run time.

    Documentation Requirements

  4. Certain aspects of the semantics are defined to be either implementation defined or unspecified. In such cases, the set of possible effects is specified, and the implementation may choose any effect in the set. Implementations shall document their behavior in implementation-defined situations, but documentation is not required for unspecified situations. The implementation-defined characteristics are summarized in Annex M.
  5. The implementation may choose to document implementation-defined behavior either by documenting what happens in general, or by providing some mechanism for the user to determine what happens in a particular case.

    Implementation Advice

  6. If an implementation detects the use of an unsupported Specialized Needs Annex feature at run time, it should raise Program_Error if feasible.
  7. If an implementation wishes to provide implementation-defined extensions to the functionality of a language-defined library unit, it should normally do so by adding children to the library unit. NOTES
  8. (2) The above requirements imply that an implementation conforming to this Standard may support some of the capabilities required by a Specialized Needs Annex without supporting all required capabilities.

Method of Description and Syntax Notation

  1. The form of an Ada program is described by means of a context-free syntax together with context-dependent requirements expressed by narrative rules.
  2. The meaning of Ada programs is described by means of narrative rules defining both the effects of each construct and the composition rules for constructs.
  3. The context-free syntax of the language is described using a simple variant of Backus-Naur Form. In particular:
    1. Lower case words in a sans-serif font, some containing embedded underlines, are used to denote syntactic categories, for example: 
    2. case_statement
    3. Boldface words are used to denote reserved words, for example: 
    4. array
    5. Square brackets enclose optional items. Thus the two following rules are equivalent. 
    6. return_statement ::= return [expression];
return_statement ::= return; | return expression;
    7. Curly brackets enclose a repeated item. The item may appear zero or more times; the repetitions occur from left to right as with an equivalent left-recursive rule. Thus the two following rules are equivalent. 
    8. term ::= factor {multiplying_operator factor}
term ::= factor | term multiplying_operator factor
    9. A vertical line separates alternative items unless it occurs immediately after an opening curly bracket, in which case it stands for itself: 
    10. constraint ::= scalar_constraint | composite_constraint
discrete_choice_list ::= discrete_choice {| discrete_choice}
    11. If the name of any syntactic category starts with an italicized part, it is equivalent to the category name without the italicized part. The italicized part is intended to convey some semantic information. For example subtype_name and task_name are both equivalent to name alone.
  1. A syntactic category is a nonterminal in the grammar defined in BNF under "Syntax." Names of syntactic categories are set in a different font, like_this.
  2. A construct is a piece of text (explicit or implicit) that is an instance of a syntactic category defined under "Syntax."
  3. A constituent of a construct is the construct itself, or any construct appearing within it.
  4. Whenever the run-time semantics defines certain actions to happen in an arbitrary order, this means that the implementation shall arrange for these actions to occur in a way that is equivalent to some sequential order, following the rules that result from that sequential order. When evaluations are defined to happen in an arbitrary order, with conversion of the results to some subtypes, or with some run-time checks, the evaluations, conversions, and checks may be arbitrarily interspersed, so long as each expression is evaluated before converting or checking its value. Note that the effect of a program can depend on the order chosen by the implementation. This can happen, for example, if two actual parameters of a given call have side effects. NOTES
  5. (3) The syntax rules describing structured constructs are presented in a form that corresponds to the recommended paragraphing. For example, an if_statement is defined as: 
  6. if_statement ::=
   if condition then
      sequence_of_statements
   {elsif condition then
      sequence_of_statements}
   [else
      sequence_of_statements]
   end if;
  7. (4) The line breaks and indentation in the syntax rules indicate the recommended line breaks and indentation in the corresponding constructs. The preferred places for other line breaks are after semicolons.

Classification of Errors

Implementation Requirements

  1. The language definition classifies errors into several different categories:
    1. Errors that are required to be detected prior to run time by every Ada implementation;
      1. These errors correspond to any violation of a rule given in this International Standard, other than those listed below. In particular, violation of any rule that uses the terms shall, allowed, permitted, legal, or illegal belongs to this category. Any program that contains such an error is not a legal Ada program; on the other hand, the fact that a program is legal does not mean, per se, that the program is free from other forms of error.
      2. The rules are further classified as either compile time rules, or post compilation rules, depending on whether a violation has to be detected at the time a compilation unit is submitted to the compiler, or may be postponed until the time a compilation unit is incorporated into a partition of a program.
    1. Errors that are required to be detected at run time by the execution of an Ada program;
      1. The corresponding error situations are associated with the names of the predefined exceptions. Every Ada compiler is required to generate code that raises the corresponding exception if such an error situation arises during program execution. If such an error situation is certain to arise in every execution of a construct, then an implementation is allowed (although not required) to report this fact at compilation time.
    1. Bounded errors;
      1. The language rules define certain kinds of errors that need not be detected either prior to or during run time, but if not detected, the range of possible effects shall be bounded. The errors of this category are called bounded errors. The possible effects of a given bounded error are specified for each such error, but in any case one possible effect of a bounded error is the raising of the exception Program_Error.
    1. Erroneous execution.
      1. In addition to bounded errors, the language rules define certain kinds of errors as leading to erroneous execution. Like bounded errors, the implementation need not detect such errors either prior to or during run time. Unlike bounded errors, there is no language-specified bound on the possible effect of erroneous execution; the effect is in general not predictable.

Implementation Permissions

  1. An implementation may provide nonstandard modes of operation. Typically these modes would be selected by a pragma or by a command line switch when the compiler is invoked. When operating in a nonstandard mode, the implementation may reject compilation_units that do not conform to additional requirements associated with the mode, such as an excessive number of warnings or violation of coding style guidelines. Similarly, in a nonstandard mode, the implementation may apply special optimizations or alternative algorithms that are only meaningful for programs that satisfy certain criteria specified by the implementation. In any case, an implementation shall support a standard mode that conforms to the requirements of this International Standard; in particular, in the standard mode, all legal compilation_units shall be accepted.

    Implementation Advice

  2. If an implementation detects a bounded error or erroneous execution, it should raise Program_Error.

Normative References

  1. The following standards contain provisions which, through reference in this text, constitute provisions of this International Standard. At the time of publication, the editions indicated were valid. All standards are subject to revision, and parties to agreements based on this International Standard are encouraged to investigate the possibility of applying the most recent editions of the standards indicated below. Members of IEC and ISO maintain registers of currently valid International Standards.
  2. ISO/IEC 646:1991, Information technology -- ISO 7-bit coded character set for information interchange.
  3. ISO/IEC 1539:1991, Information technology -- Programming languages -- FORTRAN.
  4. ISO 1989:1985, Programming languages -- COBOL.
  5. ISO/IEC 6429:1992, Information technology -- Control functions for coded graphic character sets.
  6. ISO/IEC 8859-1:1987, Information processing -- 8-bit single-byte coded character sets -- Part 1: Latin alphabet No. 1.
  7. ISO/IEC 9899:1990, Programming languages -- C.
  8. ISO/IEC 10646-1:1993, Information technology -- Universal Multiple-Octet Coded Character Set (UCS) -- Part 1: Architecture and Basic Multilingual Plane.

Definitions

  1. Terms are defined throughout this International Standard, indicated by italic type. Terms explicitly defined in this International Standard are not to be presumed to refer implicitly to similar terms defined elsewhere. Terms not defined in this International Standard are to be interpreted according to the Webster's Third New International Dictionary of the English Language. Informal descriptions of some terms are also given in Annex N, "Glossary".

Lexical Elements

  1. The text of a program consists of the texts of one or more compilations. The text of a compilation is a sequence of lexical elements, each composed of characters; the rules of composition are given in this section. Pragmas, which provide certain information for the compiler, are also described in this section.

Character Set

  1. The only characters allowed outside of comments are the graphic_characters and format_effectors.

    Syntax 

  2. character ::=
     graphic_character
   | format_effector
   | other_control_function

  3. graphic_character ::=
     identifier_letter
   | digit
   | space_character
   | special_character

    Static Semantics

  4. The character repertoire for the text of an Ada program consists of the collection of characters called the Basic Multilingual Plane (BMP) of the ISO 10646 Universal Multiple-Octet Coded Character Set, plus a set of format_ effectors and, in comments only, a set of other_control_functions; the coded representation for these characters is implementation defined (it need not be a representation defined within ISO-10646-1).
  5. The description of the language definition in this International Standard uses the graphic symbols defined for Row 00: Basic Latin and Row 00: Latin-1 Supplement of the ISO 10646 BMP; these correspond to the graphic symbols of ISO 8859-1 (Latin-1); no graphic symbols are used in this International Standard for characters outside of Row 00 of the BMP. The actual set of graphic symbols used by an implementation for the visual representation of the text of an Ada program is not specified.
  6. The categories of characters are defined as follows:
  7. identifier_letter 
    upper_case_identifier_letter | lower_case_identifier_letter
  8. upper_case_identifier_letter 
    Any character of Row 00 of ISO 10646 BMP whose name begins "Latin
Capital Letter".
  9. lower_case_identifier_letter 
    Any character of Row 00 of ISO 10646 BMP whose name begins
"Latin Small Letter".
  10. digit 
    One of the characters 0, 1, 2, 3, 4, 5, 6, 7, 8, or 9.
  11. space_character 
    The character of ISO 10646 BMP named "Space".
  12. special_character 
    Any character of the ISO 10646 BMP that is not reserved for a
control function, and is not the space_character, an
identifier_letter, or a digit.
  13. format_effector 
    The control functions of ISO 6429 called character tabulation
(HT), line tabulation (VT), carriage return (CR), line feed (LF),
and form feed (FF).
  14. other_control_function 
    Any control function, other than a format_effector, that is
allowed in a comment; the set of other_control_functions allowed
in comments is implementation defined.
  15. The following names are used when referring to certain special_characters: 
    symbol   name                   symbol   name

  "      quotation mark           :      colon
  #      number sign              ;      semicolon
  &      ampersand                <      less-than sign
  '      apostrophe, tick         =      equals sign
  (      left parenthesis         >      greater-than sign
  )      right parenthesis        _      low line, underline
  *      asterisk, multiply       |      vertical line
  +      plus sign                [      left square bracket
  ,      comma                    ]      right square bracket
  -      hyphen-minus, minus      {      left curly bracket
  .      full stop, dot, point    }      right curly bracket
  /      solidus, divide

    Implementation Permissions

  16. In a nonstandard mode, the implementation may support a different character repertoire; in particular, the set of characters that are considered identifier_letters can be extended or changed to conform to local conventions. NOTES
  17. (1) Every code position of ISO 10646 BMP that is not reserved for a control function is defined to be a graphic_character by this International Standard. This includes all code positions other than 0000 - 001F, 007F - 009F, and FFFE - FFFF.
  18. (2) The language does not specify the source representation of programs.

Lexical Elements, Separators, and Delimiters

Static Semantics

  1. The text of a program consists of the texts of one or more compilations. The text of each compilation is a sequence of separate lexical elements. Each lexical element is formed from a sequence of characters, and is either a delimiter, an identifier, a reserved word, a numeric_literal, a character_ literal, a string_literal, or a comment. The meaning of a program depends only on the particular sequences of lexical elements that form its compilations, excluding comments.
  2. The text of a compilation is divided into lines. In general, the representation for an end of line is implementation defined. However, a sequence of one or more format_effectors other than character tabulation (HT) signifies at least one end of line.
  3. In some cases an explicit separator is required to separate adjacent lexical elements. A separator is any of a space character, a format effector, or the end of a line, as follows:
    1. A space character is a separator except within a comment, a string_literal, or a character_literal.
    2. Character tabulation (HT) is a separator except within a comment.
    3. The end of a line is always a separator.
  1. One or more separators are allowed between any two adjacent lexical elements, before the first of each compilation, or after the last. At least one separator is required between an identifier, a reserved word, or a numeric_literal and an adjacent identifier, reserved word, or numeric_literal.
  2. A delimiter is either one of the following special characters 
  3. &    '    (    )    *    +    ,    -
.    /    :    ;    <    =    >    |
  4. or one of the following compound delimiters each composed of two adjacent special characters 
  5. =>    ..    **    :=    /=    >=    <=    <<    >>    <>
  6. Each of the special characters listed for single character delimiters is a single delimiter except if this character is used as a character of a compound delimiter, or as a character of a comment, string_literal, character_literal, or numeric_literal.
  7. The following names are used when referring to compound delimiters: 
  8. delimiter     name

   =>                       arrow
   ..                       double dot
   **                       double star, exponentiate
   :=                       assignment (pronounced: "becomes")
   /=                       inequality (pronounced: "not equal")
   >=                       greater than or equal
   <=                       less than or equal
   <<                       left label bracket
   >>                       right label bracket
   <>                       box

    Implementation Requirements

  9. An implementation shall support lines of at least 200 characters in length, not counting any characters used to signify the end of a line. An implementation shall support lexical elements of at least 200 characters in length. The maximum supported line length and lexical element length are implementation defined.

Identifiers

  1. Identifiers are used as names.

    Syntax 

  2. identifier ::= identifier_letter {[underline] letter_or_digit}

  3. letter_or_digit ::= identifier_letter | digit
    1. An identifier shall not be a reserved word.

Static Semantics

  1. All characters of an identifier are significant, including any underline character. Identifiers differing only in the use of corresponding upper and lower case letters are considered the same.

    Implementation Permissions

  2. In a nonstandard mode, an implementation may support other upper/lower case equivalence rules for identifiers, to accommodate local conventions.

    Examples

  3. Examples of identifiers: 
  4. Count      X    Get_Symbol   Ethelyn   Marion

Snobol_4   X1   Page_Count    Store_Next_Item

Numeric Literals

  1. There are two kinds of numeric_literals, real literals and integer literals. A real literal is a numeric_literal that includes a point; an integer literal is a numeric_literal without a point.

    Syntax 

  2. numeric_literal ::= decimal_literal | based_literal
    NOTES
  3. (3) The type of an integer literal is universal_integer. The type of a real literal is universal_real.

Decimal Literals

  1. A decimal_literal is a numeric_literal in the conventional decimal notation (that is, the base is ten).

    Syntax 

  2. decimal_literal ::= numeral [.numeral] [exponent]

  3. numeral ::= digit {[underline] digit}

  4. exponent ::= E [+] numeral | E - numeral
    1. An exponent for an integer literal shall not have a minus sign.

Static Semantics

  1. An underline character in a numeric_literal does not affect its meaning. The letter E of an exponent can be written either in lower case or in upper case, with the same meaning.
  2. An exponent indicates the power of ten by which the value of the decimal_literal without the exponent is to be multiplied to obtain the value of the decimal_literal with the exponent.

    Examples

  3. Examples of decimal literals: 
  4. 12        0      1E6    123_456           --  integer literals

12.0      0.0    0.456  3.14159_26        --  real literals

Based Literals

  1. A based_literal is a numeric_literal expressed in a form that specifies the base explicitly.

    Syntax 

  2. based_literal ::= base # based_numeral [.based_numeral] # [exponent]

  3. base ::= numeral

  4. based_numeral ::=
   extended_digit {[underline] extended_digit}

  5. extended_digit ::= digit | A | B | C | D | E | F

    Legality Rules

  6. The base (the numeric value of the decimal numeral preceding the first #) shall be at least two and at most sixteen. The extended_digits A through F represent the digits ten through fifteen, respectively. The value of each extended_digit of a based_literal shall be less than the base.

    Static Semantics

  7. The conventional meaning of based notation is assumed. An exponent indicates the power of the base by which the value of the based_literal without the exponent is to be multiplied to obtain the value of the based_literal with the exponent. The base and the exponent, if any, are in decimal notation.
  8. The extended_digits A through F can be written either in lower case or in upper case, with the same meaning.

    Examples

  9. Examples of based literals: 
  10. 2#1111_1111#   16#FF#       016#0ff#
--  integer literals of value 255

16#E#E1        2#1110_0000#
--  integer literals of value 224

16#F.FF#E+2    2#1.1111_1111_1110#E11
--  real literals of value 4095.0

Character Literals

  1. A character_literal is formed by enclosing a graphic character between two apostrophe characters.

    Syntax 

  2. character_literal ::= 'graphic_character'
    NOTES
  3. (4) A character_literal is an enumeration literal of a character type. (see section Character Types).

    Examples

  4. Examples of character literals: 
  5. 'A'     '*'     "'     ' '

String Literals

  1. A string_literal is formed by a sequence of graphic characters (possibly none) enclosed between two quotation marks used as string brackets. They are used to represent operator_symbols (see section Subprogram Declarations) values of a string type (see section Literals) and array subaggregates (see section Array Aggregates).

    Syntax 

  2. string_literal ::= "{string_element}"

  3. string_element ::= "" | non_quotation_mark_graphic_character
    1. A string_element is either a pair of quotation marks (""), or a single graphic_character other than a quotation mark.

Static Semantics

  1. The sequence of characters of a string_literal is formed from the sequence of string_elements between the bracketing quotation marks, in the given order, with a string_element that is "" becoming a single quotation mark in the sequence of characters, and any other string_element being reproduced in the sequence.
  2. A null string literal is a string_literal with no string_elements between the quotation marks. NOTES
  3. (5) An end of line cannot appear in a string_literal.

    Examples

  4. Examples of string literals: 
  5. 
"Message of the day:"

""                           --  a null string literal
" "   "A"   """"             --  three string literals of length 1

"Characters such as $, %, and } are allowed in string literals"

Comments

  1. A comment starts with two adjacent hyphens and extends up to the end of the line.

    Syntax 

  2. comment ::= --{non_end_of_line_character}
    1. A comment may appear on any line of a program.

Static Semantics

  1. The presence or absence of comments has no influence on whether a program is legal or illegal. Furthermore, comments do not influence the meaning of a program; their sole purpose is the enlightenment of the human reader.

    Examples

  2. Examples of comments: 
  3. --  the last sentence above echoes the Algol 68 report

end;  --  processing of Line is complete

--  a long comment may be split onto
--  two or more consecutive lines

----------------  the first two hyphens start the comment

Pragmas

  1. A pragma is a compiler directive. There are language-defined pragmas that give instructions for optimization, listing control, etc. An implementation may support additional (implementation-defined) pragmas.

    Syntax 

  2. pragma ::=
   pragma identifier [(pragma_argument_association
      {, pragma_argument_association})];

  3. pragma_argument_association ::=
     [pragma_argument_identifier =>] name
   | [pragma_argument_identifier =>] expression
    1. In a pragma, any pragma_argument_associations without a pragma_argument_identifier shall precede any associations with a pragma_argument_identifier.
    2. Pragmas are only allowed at the following places in a program:
      1. After a semicolon delimiter, but not within a formal_part or discriminant_part.
      2. At any place where the syntax rules allow a construct defined by a syntactic category whose name ends with "declaration", "statement", "clause", or "alternative", or one of the syntactic categories variant or exception_handler; but not in place of such a construct. Also at any place where a compilation_unit would be allowed.
    1. Additional syntax rules and placement restrictions exist for specific pragmas.
  1. The name of a pragma is the identifier following the reserved word pragma. The name or expression of a pragma_argument_association is a pragma argument.
  2. An identifier specific to a pragma is an identifier that is used in a pragma argument with special meaning for that pragma.

    Static Semantics

  3. If an implementation does not recognize the name of a pragma, then it has no effect on the semantics of the program. Inside such a pragma, the only rules that apply are the Syntax Rules.

    Dynamic Semantics

  4. Any pragma that appears at the place of an executable construct is executed. Unless otherwise specified for a particular pragma, this execution consists of the evaluation of each evaluable pragma argument in an arbitrary order.

    Implementation Requirements

  5. The implementation shall give a warning message for an unrecognized pragma name.

    Implementation Permissions

  6. An implementation may provide implementation-defined pragmas; the name of an implementation-defined pragma shall differ from those of the language-defined pragmas.
  7. An implementation may ignore an unrecognized pragma even if it violates some of the Syntax Rules, if detecting the syntax error is too complex.

    Implementation Advice

  8. Normally, implementation-defined pragmas should have no semantic effect for error-free programs; that is, if the implementation-defined pragmas are removed from a working program, the program should still be legal, and should still have the same semantics.
  9. Normally, an implementation should not define pragmas that can make an illegal program legal, except as follows:
    1. A pragma used to complete a declaration, such as a pragma Import;
    2. A pragma used to configure the environment by adding, removing, or replacing library_items.

Syntax

  1. The forms of List, Page, and Optimize pragmas are as follows:
  2. pragma List(identifier);
  3. pragma Page;
  4. pragma Optimize(identifier);
    1. Other pragmas are defined throughout this International Standard, and are summarized in Annex L.

Static Semantics

  1. A pragma List takes one of the identifiers On or Off as the single argument. This pragma is allowed anywhere a pragma is allowed. It specifies that listing of the compilation is to be continued or suspended until a List pragma with the opposite argument is given within the same compilation. The pragma itself is always listed if the compiler is producing a listing.
  2. A pragma Page is allowed anywhere a pragma is allowed. It specifies that the program text which follows the pragma should start on a new page (if the compiler is currently producing a listing).
  3. A pragma Optimize takes one of the identifiers Time, Space, or Off as the single argument. This pragma is allowed anywhere a pragma is allowed, and it applies until the end of the immediately enclosing declarative region, or for a pragma at the place of a compilation_unit, to the end of the compilation. It gives advice to the implementation as to whether time or space is the primary optimization criterion, or that optional optimizations should be turned off. It is implementation defined how this advice is followed.

    Examples

  4. Examples of pragmas: 
  5. pragma List(Off);         -- turn off listing generation
pragma Optimize(Off);     -- turn off optional optimizations
pragma Inline(Set_Mask);  -- generate code for Set_Mask inline

pragma Suppress(Range_Check, On => Index);
-- turn off range checking on Index

Reserved Words

Syntax

    1. The following are the reserved words (ignoring upper/lower case distinctions): 
      abort          else           new            return
abs            elsif          not            reverse
abstract       end            null
accept         entry                         select
access         exception                     separate
aliased        exit           of             subtype
all                           or
and            for            others         tagged
array          function       out            task
at                                           terminate
               generic        package        then
begin          goto           pragma         type
body                          private
               if             procedure
case           in             protected      until
constant       is                            use
                              raise
declare                       range          when
delay          limited        record         while
delta          loop           rem            with
digits                        renames
do             mod            requeue        xor
    NOTES
  1. (6) The reserved words appear in lower case boldface in this International Standard, except when used in the designator of an attribute (see section Attributes). Lower case boldface is also used for a reserved word in a string_literal used as an operator_symbol. This is merely a convention -- programs may be written in whatever typeface is desired and available.

Declarations and Types

  1. This section describes the types in the language and the rules for declaring constants, variables, and named numbers.

Declarations

  1. The language defines several kinds of named entities that are declared by declarations. The entity's name is defined by the declaration, usually by a defining_identifier, but sometimes by a defining_character_literal or defining_operator_symbol.
  2. There are several forms of declaration. A basic_declaration is a form of declaration defined as follows.

    Syntax 

  3. basic_declaration ::=
     type_declaration         | subtype_declaration
   | object_declaration       | number_declaration
   | subprogram_declaration   | abstract_subprogram_declaration
   | package_declaration      | renaming_declaration
   | exception_declaration    | generic_declaration
   | generic_instantiation

  4. defining_identifier ::= identifier

    Static Semantics

  5. A declaration is a language construct that associates a name with (a view of) an entity. A declaration may appear explicitly in the program text (an explicit declaration), or may be supposed to occur at a given place in the text as a consequence of the semantics of another construct (an implicit declaration).
  6. Each of the following is defined to be a declaration: any basic_declaration; an enumeration_literal_specification; a discriminant_specification; a component_declaration; a loop_parameter_specification; a parameter_specification; a subprogram_body; an entry_declaration; an entry_index_specification; a choice_parameter_specification; a generic_formal_parameter_declaration.
  7. All declarations contain a definition for a view of an entity. A view consists of an identification of the entity (the entity of the view), plus view-specific characteristics that affect the use of the entity through that view (such as mode of access to an object, formal parameter names and defaults for a subprogram, or visibility to components of a type). In most cases, a declaration also contains the definition for the entity itself (a renaming_declaration is an example of a declaration that does not define a new entity, but instead defines a view of an existing entity, see section Renaming Declarations.)
  8. For each declaration, the language rules define a certain region of text called the scope of the declaration (see section Scope of Declarations). Most declarations associate an identifier with a declared entity. Within its scope, and only there, there are places where it is possible to use the identifier to refer to the declaration, the view it defines, and the associated entity; these places are defined by the visibility rules (see section Visibility). At such places the identifier is said to be a name of the entity (the direct_name or selector_name); the name is said to denote the declaration, the view, and the associated entity (see section The Context of Overload Resolution). The declaration is said to declare the name, the view, and in most cases, the entity itself.
  9. As an alternative to an identifier, an enumeration literal can be declared with a character_literal as its name (see section Enumeration Types) and a function can be declared with an operator_symbol as its name (see section Subprogram Declarations).
  10. The syntax rules use the terms defining_identifier, defining_character_literal, and defining_operator_symbol for the defining occurrence of a name; these are collectively called defining names. The terms direct_name and selector_name are used for usage occurrences of identifiers, character_literals, and operator_symbols. These are collectively called usage names.

    Dynamic Semantics

  11. The process by which a construct achieves its run-time effect is called execution. This process is also called elaboration for declarations and evaluation for expressions. One of the terms execution, elaboration, or evaluation is defined by this International Standard for each construct that has a run-time effect. NOTES
  12. (1) At compile time, the declaration of an entity declares the entity. At run time, the elaboration of the declaration creates the entity.

Types and Subtypes

Static Semantics

  1. A type is characterized by a set of values, and a set of primitive operations which implement the fundamental aspects of its semantics. An object of a given type is a run-time entity that contains (has) a value of the type.
  2. Types are grouped into classes of types, reflecting the similarity of their values and primitive operations. There exist several language-defined classes of types (see NOTES below). Elementary types are those whose values are logically indivisible; composite types are those whose values are composed of component values.
  3. The elementary types are the scalar types (discrete and real) and the access types (whose values provide access to objects or subprograms). Discrete types are either integer types or are defined by enumeration of their values (enumeration types). Real types are either floating point types or fixed point types.
  4. The composite types are the record types, record extensions, array types, task types, and protected types. A private type or private extension represents a partial view, see section Private Types and Private Extensions, of a type, providing support for data abstraction. A partial view is a composite type.
  5. Certain composite types (and partial views thereof) have special components called discriminants whose values affect the presence, constraints, or initialization of other components. Discriminants can be thought of as parameters of the type.
  6. The term subcomponent is used in this International Standard in place of the term component to indicate either a component, or a component of another subcomponent. Where other subcomponents are excluded, the term component is used instead. Similarly, a part of an object or value is used to mean the whole object or value, or any set of its subcomponents.
  7. The set of possible values for an object of a given type can be subjected to a condition that is called a constraint (the case of a null constraint that specifies no restriction is also included); the rules for which values satisfy a given kind of constraint are given in section Scalar Types for range_constraints, section Index Constraints and Discrete Ranges, for index_constraints, and section Discriminant Constraints, for discriminant_constraints.
  8. A subtype of a given type is a combination of the type, a constraint on values of the type, and certain attributes specific to the subtype. The given type is called the type of the subtype. Similarly, the associated constraint is called the constraint of the subtype. The set of values of a subtype consists of the values of its type that satisfy its constraint. Such values belong to the subtype.
  9. A subtype is called an unconstrained subtype if its type has unknown discriminants, or if its type allows range, index, or discriminant constraints, but the subtype does not impose such a constraint; otherwise, the subtype is called a constrained subtype (since it has no unconstrained characteristics). NOTES
  10. (2) Any set of types that is closed under derivation (see section Derived Types and Classes) can be called a "class" of types. However, only certain classes are used in the description of the rules of the language -- generally those that have their own particular set of primitive operations (see section Classification of Operations) or that correspond to a set of types that are matched by a given kind of generic formal type (see section Formal Types). The following are examples of "interesting" language-defined classes: elementary, scalar, discrete, enumeration, character, boolean, integer, signed integer, modular, real, floating point, fixed point, ordinary fixed point, decimal fixed point, numeric, access, access-to-object, access-to-subprogram, composite, array, string, (untagged) record, tagged, task, protected, nonlimited. Special syntax is provided to define types in each of these classes.
    1. These language-defined classes are organized like this: 
    2. all types
   elementary
      scalar
         discrete
            enumeration
               character
               boolean
               other enumeration
            integer
               signed integer
               modular integer
         real
            floating point
            fixed point
               ordinary fixed point
               decimal fixed point
      access
         access-to-object
         access-to-subprogram
   composite
      array
         string
         other array
      untagged record
      tagged
      task
      protected
    3. The classes "numeric" and "nonlimited" represent other classification dimensions and do not fit into the above strictly hierarchical picture.

Type Declarations

  1. A type_declaration declares a type and its first subtype.

    Syntax 

  2. type_declaration ::=
     full_type_declaration
   | incomplete_type_declaration
   | private_type_declaration
   | private_extension_declaration

  3. full_type_declaration ::=
     type defining_identifier [known_discriminant_part]
       is type_definition;
   | task_type_declaration
   | protected_type_declaration

  4. type_definition ::=
     enumeration_type_definition | integer_type_definition
   | real_type_definition        | array_type_definition
   | record_type_definition      | access_type_definition
   | derived_type_definition

    Legality Rules

  5. A given type shall not have a subcomponent whose type is the given type itself.

    Static Semantics

  6. The defining_identifier of a type_declaration denotes the first subtype of the type. The known_discriminant_part, if any, defines the discriminants of the type (see section Discriminants). The remainder of the type_ declaration defines the remaining characteristics of (the view of) the type.
  7. A type defined by a type_declaration is a named type; such a type has one or more nameable subtypes. Certain other forms of declaration also include type definitions as part of the declaration for an object (including a parameter or a discriminant). The type defined by such a declaration is anonymous -- it has no nameable subtypes. For explanatory purposes, this International Standard sometimes refers to an anonymous type by a pseudo-name, written in italics, and uses such pseudo-names at places where the syntax normally requires an identifier. For a named type whose first subtype is T, this International Standard sometimes refers to the type of T as simply "the type T."
  8. A named type that is declared by a full_type_declaration, or an anonymous type that is defined as part of declaring an object of the type, is called a full type. The type_definition, task_definition, protected_definition, or access_definition that defines a full type is called a full type definition. Types declared by other forms of type_declaration are not separate types; they are partial or incomplete views of some full type.
  9. The definition of a type implicitly declares certain predefined operators that operate on the type, according to what classes the type belongs, as specified in section Operators and Expression Evaluation.
  10. The predefined types (for example the types Boolean, Wide_Character, Integer, root_integer, and universal_integer) are the types that are defined in a predefined library package called Standard; this package also includes the (implicit) declarations of their predefined operators. The package Standard is described in section The Package Standard.

    Dynamic Semantics

  11. The elaboration of a full_type_declaration consists of the elaboration of the full type definition. Each elaboration of a full type definition creates a distinct type and its first subtype.

    Examples

  12. Examples of type definitions: 
  13. (White, Red, Yellow, Green, Blue, Brown, Black)
range 1 .. 72
array(1 .. 10) of Integer
  14. Examples of type declarations: 
  15. type Color  is (White, Red, Yellow, Green, Blue, Brown, Black);
type Column is range 1 .. 72;
type Table  is array(1 .. 10) of Integer;
    NOTES
  16. (3) Each of the above examples declares a named type. The identifier given denotes the first subtype of the type. Other named subtypes of the type can be declared with subtype_declarations (see section Subtype Declarations). Although names do not directly denote types, a phrase like "the type Column" is sometimes used in this International Standard to refer to the type of Column, where Column denotes the first subtype of the type. For an example of the definition of an anonymous type, see the declaration of the array Color_Table in section Object Declarations, its type is anonymous -- it has no nameable subtypes.

Subtype Declarations

  1. A subtype_declaration declares a subtype of some previously declared type, as defined by a subtype_indication.

    Syntax 

  2. subtype_declaration ::=
   subtype defining_identifier is subtype_indication;

  3. subtype_indication ::=  subtype_mark [constraint]

  4. subtype_mark ::= subtype_name

  5. constraint ::= scalar_constraint | composite_constraint

  6. scalar_constraint ::=
   range_constraint | digits_constraint | delta_constraint

  7. composite_constraint ::=
   index_constraint | discriminant_constraint

    Name Resolution Rules

  8. A subtype_mark shall resolve to denote a subtype. The type determined by a subtype_mark is the type of the subtype denoted by the subtype_mark.

    Dynamic Semantics

  9. The elaboration of a subtype_declaration consists of the elaboration of the subtype_indication. The elaboration of a subtype_indication creates a new subtype. If the subtype_indication does not include a constraint, the new subtype has the same (possibly null) constraint as that denoted by the subtype_mark. The elaboration of a subtype_indication that includes a constraint proceeds as follows:
    1. The constraint is first elaborated.
    2. A check is then made that the constraint is compatible with the subtype denoted by the subtype_mark.
  1. The condition imposed by a constraint is the condition obtained after elaboration of the constraint. The rules defining compatibility are given for each form of constraint in the appropriate subclause. These rules are such that if a constraint is compatible with a subtype, then the condition imposed by the constraint cannot contradict any condition already imposed by the subtype on its values. The exception Constraint_Error is raised if any check of compatibility fails. NOTES
  2. (4) A scalar_constraint may be applied to a subtype of an appropriate scalar type, see section Scalar Types, see section Fixed Point Types, and section Reduced Accuracy Subtypes, even if the subtype is already constrained. On the other hand, a composite_constraint may be applied to a composite subtype (or an access-to-composite subtype) only if the composite subtype is unconstrained, see section Index Constraints and Discrete Ranges and section Discriminant Constraints.

    Examples

  3. Examples of subtype declarations: 
  4. subtype Rainbow   is Color range Red .. Blue;  -- see section Type Declarations
subtype Red_Blue  is Rainbow;
subtype Int       is Integer;
subtype Small_Int is Integer range -10 .. 10;
subtype Up_To_K   is Column range 1 .. K;      -- see section Type Declarations
subtype Square    is Matrix(1 .. 10, 1 .. 10); -- see section Array Types
subtype Male      is Person(Sex => M);         -- see section Incomplete Type Declarations

Classification of Operations

Static Semantics

  1. An operation operates on a type T if it yields a value of type T, if it has an operand whose expected type, see section The Context of Overload Resolution, is T, or if it has an access parameter, see section Subprogram Declarations designating T. A predefined operator, or other language-defined operation such as assignment or a membership test, that operates on a type, is called a predefined operation of the type. The primitive operations of a type are the predefined operations of the type, plus any user-defined primitive subprograms.
  2. The primitive subprograms of a specific type are defined as follows:
    1. The predefined operators of the type, see section Operators and Expression Evaluation,
    2. For a derived type, the inherited, see section Derived Types and Classes, user-defined subprograms;
    3. For an enumeration type, the enumeration literals (which are considered parameterless functions -- see section Enumeration Types.);
    4. For a specific type declared immediately within a package_specification, any subprograms (in addition to the enumeration literals) that are explicitly declared immediately within the same package_specification and that operate on the type;
    5. Any subprograms not covered above that are explicitly declared immediately within the same declarative region as the type and that override, see section Visibility, other implicitly declared primitive subprograms of the type.
  1. A primitive subprogram whose designator is an operator_symbol is called a primitive operator.

Objects and Named Numbers

  1. Objects are created at run time and contain a value of a given type. An object can be created and initialized as part of elaborating a declaration, evaluating an allocator, aggregate, or function_call, or passing a parameter by copy. Prior to reclaiming the storage for an object, it is finalized if necessary, see section Completion and Finalization.

    Static Semantics

  2. All of the following are objects:
    1. the entity declared by an object_declaration;
    2. a formal parameter of a subprogram, entry, or generic subprogram;
    3. a generic formal object;
    4. a loop parameter;
    5. a choice parameter of an exception_handler;
    6. an entry index of an entry_body;
    7. the result of dereferencing an access-to-object value, see section Names,
    8. the result of evaluating a function_call (or the equivalent operator invocation -- see section Overloading of Operators,
    9. the result of evaluating an aggregate;
    10. a component, slice, or view conversion of another object.
  1. An object is either a constant object or a variable object. The value of a constant object cannot be changed between its initialization and its finalization, whereas the value of a variable object can be changed. Similarly, a view of an object is either a constant or a variable. All views of a constant object are constant. A constant view of a variable object cannot be used to modify the value of the variable. The terms constant and variable by themselves refer to constant and variable views of objects.
  2. The value of an object is read when the value of any part of the object is evaluated, or when the value of an enclosing object is evaluated. The value of a variable is updated when an assignment is performed to any part of the variable, or when an assignment is performed to an enclosing object.
  3. Whether a view of an object is constant or variable is determined by the definition of the view. The following (and no others) represent constants:
    1. an object declared by an object_declaration with the reserved word constant;
    2. a formal parameter or generic formal object of mode in;
    3. a discriminant;
    4. a loop parameter, choice parameter, or entry index;
    5. the dereference of an access-to-constant value;
    6. he result of evaluating a function_call or an aggregate;
    7. a selected_component, indexed_component, slice, or view conversion of a constant.
  1. At the place where a view of an object is defined, a nominal subtype is associated with the view. The object's actual subtype (that is, its subtype) can be more restrictive than the nominal subtype of the view; it always is if the nominal subtype is an indefinite subtype. A subtype is an indefinite subtype if it is an unconstrained array subtype, or if it has unknown discriminants or unconstrained discriminants without defaults (see section Discriminants) otherwise the subtype is a definite subtype (all elementary subtypes are definite subtypes). A class-wide subtype is defined to have unknown discriminants, and is therefore an indefinite subtype. An indefinite subtype does not by itself provide enough information to create an object; an additional constraint or explicit initialization expression is necessary (see section Object Declarations). A component cannot have an indefinite nominal subtype.
  2. A named number provides a name for a numeric value known at compile time. It is declared by a number_declaration. NOTES
  3. (5) A constant cannot be the target of an assignment operation, nor be passed as an in out or out parameter, between its initialization and finalization, if any.
  4. (6) The nominal and actual subtypes of an elementary object are always the same. For a discriminated or array object, if the nominal subtype is constrained then so is the actual subtype.

Object Declarations

  1. An object_declaration declares a stand-alone object with a given nominal subtype and, optionally, an explicit initial value given by an initialization expression. For an array, task, or protected object, the object_declaration may include the definition of the (anonymous) type of the object.

    Syntax 

  2. object_declaration ::=
    defining_identifier_list : [aliased] [constant]
      subtype_indication [:= expression];
  | defining_identifier_list : [aliased] [constant]
      array_type_definition [:= expression];
  | single_task_declaration
  | single_protected_declaration

  3. defining_identifier_list ::=
   defining_identifier {, defining_identifier}

    Name Resolution Rules

  4. For an object_declaration with an expression following the compound delimiter :=, the type expected for the expression is that of the object. This expression is called the initialization expression.

    Legality Rules

  5. An object_declaration without the reserved word constant declares a variable object. If it has a subtype_indication or an array_type_definition that defines an indefinite subtype, then there shall be an initialization expression. An initialization expression shall not be given if the object is of a limited type.

    Static Semantics

  6. An object_declaration with the reserved word constant declares a constant object. If it has an initialization expression, then it is called a full constant declaration. Otherwise it is called a deferred constant declaration. The rules for deferred constant declarations are given in clause (see section Deferred Constants). The rules for full constant declarations are given in this subclause.
  7. Any declaration that includes a defining_identifier_list with more than one defining_identifier is equivalent to a series of declarations each containing one defining_identifier from the list, with the rest of the text of the declaration copied for each declaration in the series, in the same order as the list. The remainder of this International Standard relies on this equivalence; explanations are given for declarations with a single defining_identifier.
  8. The subtype_indication or full type definition of an object_declaration defines the nominal subtype of the object. The object_declaration declares an object of the type of the nominal subtype.

    Dynamic Semantics

  9. If a composite object declared by an object_declaration has an unconstrained nominal subtype, then if this subtype is indefinite or the object is constant or aliased, see section Access Types, the actual subtype of this object is constrained. The constraint is determined by the bounds or discriminants (if any) of its initial value; the object is said to be constrained by its initial value. In the case of an aliased object, this initial value may be either explicit or implicit; in the other cases, an explicit initial value is required. When not constrained by its initial value, the actual and nominal subtypes of the object are the same. If its actual subtype is constrained, the object is called a constrained object.
  10. For an object_declaration without an initialization expression, any initial values for the object or its subcomponents are determined by the implicit initial values defined for its nominal subtype, as follows:
    1. The implicit initial value for an access subtype is the null value of the access type.
    2. The implicit initial (and only) value for each discriminant of a constrained discriminated subtype is defined by the subtype.
    3. For a (definite) composite subtype, the implicit initial value of each component with a default_expression is obtained by evaluation of this expression and conversion to the component's nominal subtype (which might raise Constraint_Error -- see section Type Conversions.), unless the component is a discriminant of a constrained subtype (the previous case), or is in an excluded variant, see section Variant Parts and Discrete Choices. For each component that does not have a default_expression, any implicit initial values are those determined by the component's nominal subtype.
    4. For a protected or task subtype, there is an implicit component (an entry queue) corresponding to each entry, with its implicit initial value being an empty queue.
  1. The elaboration of an object_declaration proceeds in the following sequence of steps:
    1. The subtype_indication, array_type_definition, single_task_declaration, or single_protected_declaration is first elaborated. This creates the nominal subtype (and the anonymous type in the latter three cases).
    2. If the object_declaration includes an initialization expression, the (explicit) initial value is obtained by evaluating the expression and converting it to the nominal subtype (which might raise Constraint_Error -- see section Type Conversions.).
    3. The object is created, and, if there is not an initialization expression, any per-object expressions, see section Record Types are evaluated and any implicit initial values for the object or for its subcomponents are obtained as determined by the nominal subtype.
    4. Any initial values (whether explicit or implicit) are assigned to the object or to the corresponding subcomponents. As described in section Assignment Statements, and section User-Defined Assignment and Finalization, Initialize and Adjust procedures can be called.
  1. For the third step above, the object creation and any elaborations and evaluations are performed in an arbitrary order, except that if the default_expression for a discriminant is evaluated to obtain its initial value, then this evaluation is performed before that of the default_expression for any component that depends on the discriminant, and also before that of any default_expression that includes the name of the discriminant. The evaluations of the third step and the assignments of the fourth step are performed in an arbitrary order, except that each evaluation is performed before the resulting value is assigned.
  2. There is no implicit initial value defined for a scalar subtype. In the absence of an explicit initialization, a newly created scalar object might have a value that does not belong to its subtype (see section Data Validity and section Pragma Normalize_Scalars). NOTES
  3. (7) Implicit initial values are not defined for an indefinite subtype, because if an object's nominal subtype is indefinite, an explicit initial value is required.
  4. (8) As indicated above, a stand-alone object is an object declared by an object_declaration. Similar definitions apply to "stand-alone constant" and "stand-alone variable." A subcomponent of an object is not a stand-alone object, nor is an object that is created by an allocator. An object declared by a loop_parameter_specification, parameter_specification, entry_index_specification, choice_parameter_specification, or a formal_object_declaration is not called a stand-alone object.
  5. (9) The type of a stand-alone object cannot be abstract, see section Abstract Types and Subprograms.

    Examples

  6. Example of a multiple object declaration: 
  7. --  the multiple object declaration

  8. John, Paul : Person_Name := new Person(Sex => M); --  see section Incomplete Type Declarations

  9. --  is equivalent to the two single object
--  declarations in the order given

  10. John : Person_Name := new Person(Sex => M);
Paul : Person_Name := new Person(Sex => M);
  11. Examples of variable declarations: 
  12. Count, Sum  : Integer;
Size        : Integer range 0 .. 10_000 := 0;
Sorted      : Boolean := False;
Color_Table : array(1 .. Max) of Color;
Option      : Bit_Vector(1 .. 10) := (others => True);
Hello       : constant String := "Hi, world.";
  13. Examples of constant declarations: 
  14. Limit     : constant Integer := 10_000;
Low_Limit : constant Integer := Limit/10;
Tolerance : constant Real := Dispersion(1.15);

Number Declarations

  1. A number_declaration declares a named number.

    Syntax 

  2. number_declaration ::=
   defining_identifier_list : constant := static_expression;

    Name Resolution Rules

  3. The static_expression given for a number_declaration is expected to be of any numeric type.

    Legality Rules

  4. The static_expression given for a number declaration shall be a static expression, as defined by clause (see section Static Expressions and Static Subtypes).

    Static Semantics

  5. The named number denotes a value of type universal_integer if the type of the static_expression is an integer type. The named number denotes a value of type universal_real if the type of the static_expression is a real type.
  6. The value denoted by the named number is the value of the static_expression, converted to the corresponding universal type.

    Dynamic Semantics

  7. The elaboration of a number_declaration has no effect.

    Examples

  8. Examples of number declarations: 
  9. Two_Pi        : constant := 2.0*Ada.Numerics.Pi;
-- a real number, see section The Numerics Packages

  10. Max           : constant := 500;     -- an integer number
Max_Line_Size : constant := Max/6;   -- the integer 83
Power_16      : constant := 2**16;   -- the integer 65_536
One, Un, Eins : constant := 1;       -- three different names for 1

Derived Types and Classes

  1. A derived_type_definition defines a new type (and its first subtype) whose characteristics are derived from those of a parent type.

    Syntax 

  2. derived_type_definition ::= [abstract] new
  parent_subtype_indication [record_extension_part]

    Legality Rules

  3. The parent_subtype_indication defines the parent subtype; its type is the parent type.
  4. A type shall be completely defined, see section Completions of Declarations, prior to being specified as the parent type in a derived_type_definition -- the full_type_declarations for the parent type and any of its subcomponents have to precede the derived_type_definition.
  5. If there is a record_extension_part, the derived type is called a record extension of the parent type. A record_extension_part shall be provided if and only if the parent type is a tagged type.

    Static Semantics

  6. The first subtype of the derived type is unconstrained if a known_discriminant_part is provided in the declaration of the derived type, or if the parent subtype is unconstrained. Otherwise, the constraint of the first subtype corresponds to that of the parent subtype in the following sense: it is the same as that of the parent subtype except that for a range constraint (implicit or explicit), the value of each bound of its range is replaced by the corresponding value of the derived type.
  7. The characteristics of the derived type are defined as follows:
    1. Each class of types that includes the parent type also includes the derived type.
    2. If the parent type is an elementary type or an array type, then the set of possible values of the derived type is a copy of the set of possible values of the parent type. For a scalar type, the base range of the derived type is the same as that of the parent type.
    3. If the parent type is a composite type other than an array type, then the components, protected subprograms, and entries that are declared for the derived type are as follows:
      1. The discriminants specified by a new known_discriminant_part, if there is one; otherwise, each discriminant of the parent type (implicitly declared in the same order with the same specifications) -- in the latter case, the discriminants are said to be inherited, or if unknown in the parent, are also unknown in the derived type;
      2. Each nondiscriminant component, entry, and protected subprogram of the parent type, implicitly declared in the same order with the same declarations; these components, entries, and protected subprograms are said to be inherited;
      3. Each component declared in a record_extension_part, if any.
    1. Declarations of components, protected subprograms, and entries, whether implicit or explicit, occur immediately within the declarative region of the type, in the order indicated above, following the parent subtype_indication.
    2. The derived type is limited if and only if the parent type is limited.
    3. For each predefined operator of the parent type, there is a corresponding predefined operator of the derived type.
    4. For each user-defined primitive subprogram (other than a user-defined equality operator -- see below) of the parent type that already exists at the place of the derived_type_definition, there exists a corresponding inherited primitive subprogram of the derived type with the same defining name. Primitive user-defined equality operators of the parent type are also inherited by the derived type, except when the derived type is a nonlimited record extension, and the inherited operator would have a profile that is type conformant with the profile of the corresponding predefined equality operator; in this case, the user-defined equality operator is not inherited, but is rather incorporated into the implementation of the predefined equality operator of the record extension, see section Relational Operators and Membership Tests.
    5. The profile of an inherited subprogram (including an inherited enumeration literal) is obtained from the profile of the corresponding (user-defined) primitive subprogram of the parent type, after systematic replacement of each subtype of its profile, see section Subprogram Declarations, that is of the parent type with a corresponding subtype of the derived type. For a given subtype of the parent type, the corresponding subtype of the derived type is defined as follows:
      1. If the declaration of the derived type has neither a known_discriminant_part nor a record_extension_part, then the corresponding subtype has a constraint that corresponds (as defined above for the first subtype of the derived type) to that of the given subtype.
      2. If the derived type is a record extension, then the corresponding subtype is the first subtype of the derived type.
      3. If the derived type has a new known_discriminant_part but is not a record extension, then the corresponding subtype is constrained to those values that when converted to the parent type belong to the given subtype, see section Type Conversions.
    1. The same formal parameters have default_expressions in the profile of the inherited subprogram. Any type mismatch due to the systematic replacement of the parent type by the derived type is handled as part of the normal type conversion associated with parameter passing -- see section Parameter Associations.
  1. If a primitive subprogram of the parent type is visible at the place of the derived_type_definition, then the corresponding inherited subprogram is implicitly declared immediately after the derived_type_definition. Otherwise, the inherited subprogram is implicitly declared later or not at all, as explained in section Private Operations.
  2. A derived type can also be defined by a private_extension_declaration (see section Private Types and Private Extensions) or a formal_derived_type_definition, see section Formal Private and Derived Types. Such a derived type is a partial view of the corresponding full or actual type.
  3. All numeric types are derived types, in that they are implicitly derived from a corresponding root numeric type (see section Integer Types, and section Real Types).

    Dynamic Semantics

  4. The elaboration of a derived_type_definition creates the derived type and its first subtype, and consists of the elaboration of the subtype_indication and the record_extension_part, if any. If the subtype_indication depends on a discriminant, then only those expressions that do not depend on a discriminant are evaluated.
  5. For the execution of a call on an inherited subprogram, a call on the corresponding primitive subprogram of the parent type is performed; the normal conversion of each actual parameter to the subtype of the corresponding formal parameter, see section Parameter Associations performs any necessary type conversion as well. If the result type of the inherited subprogram is the derived type, the result of calling the parent's subprogram is converted to the derived type. NOTES
  6. (10) Classes are closed under derivation -- any class that contains a type also contains its derivatives. Operations available for a given class of types are available for the derived types in that class.
  7. (11) Evaluating an inherited enumeration literal is equivalent to evaluating the corresponding enumeration literal of the parent type, and then converting the result to the derived type. This follows from their equivalence to parameterless functions.
  8. (12) A generic subprogram is not a subprogram, and hence cannot be a primitive subprogram and cannot be inherited by a derived type. On the other hand, an instance of a generic subprogram can be a primitive subprogram, and hence can be inherited.
  9. (13) If the parent type is an access type, then the parent and the derived type share the same storage pool; there is a null access value for the derived type and it is the implicit initial value for the type (see section Access Types).
  10. (14) If the parent type is a boolean type, the predefined relational operators of the derived type deliver a result of the predefined type Boolean, see section Relational Operators and Membership Tests. If the parent type is an integer type, the right operand of the predefined exponentiation operator is of the predefined type Integer, see section Highest Precedence Operators.
  11. (15) Any discriminants of the parent type are either all inherited, or completely replaced with a new set of discriminants.
  12. (16) For an inherited subprogram, the subtype of a formal parameter of the derived type need not have any value in common with the first subtype of the derived type.
  13. (17) If the reserved word abstract is given in the declaration of a type, the type is abstract, see section Abstract Types and Subprograms.

    Examples

  14. Examples of derived type declarations: 
  15. type Local_Coordinate is new Coordinate;   --  two different types
type Midweek is new Day range Tue .. Thu;  --  see section Enumeration Types
type Counter is new Positive;              --  same range as Positive

  16. type Special_Key is new Key_Manager.Key;   --  see section Private Operations
  -- the inherited subprograms have the following specifications:
  --         procedure Get_Key(K : out Special_Key);
  --         function "<"(X,Y : Special_Key) return Boolean;

Derivation Classes

  1. In addition to the various language-defined classes of types, types can be grouped into derivation classes.

    Static Semantics

  2. A derived type is derived from its parent type directly; it is derived indirectly from any type from which its parent type is derived. The derivation class of types for a type T (also called the class rooted at T) is the set consisting of T (the root type of the class) and all types derived from T (directly or indirectly) plus any associated universal or class-wide types (defined below).
  3. Every type is either a specific type, a class-wide type, or a universal type. A specific type is one defined by a type_declaration, a formal_type_declaration, or a full type definition embedded in a declaration for an object. Class-wide and universal types are implicitly defined, to act as representatives for an entire class of types, as follows:
  4. Class-wide types
      Class-wide types are defined for (and belong to) each derivation class rooted at a tagged type, see section Tagged Types and Type Extensions. Given a subtype S of a tagged type T, S'Class is the subtype_mark for a corresponding subtype of the tagged class-wide type T'Class. Such types are called "class-wide" because when a formal parameter is defined to be of a class-wide type T'Class, an actual parameter of any type in the derivation class rooted at T is acceptable, see section The Context of Overload Resolution.
    1. The set of values for a class-wide type T'Class is the discriminated union of the set of values of each specific type in the derivation class rooted at T (the tag acts as the implicit discriminant -- see section Tagged Types and Type Extensions.). Class-wide types have no primitive subprograms of their own. However, as explained in section Dispatching Operations of Tagged Types, operands of a class-wide type T'Class can be used as part of a dispatching call on a primitive subprogram of the type T. The only components (including discriminants) of T'Class that are visible are those of T. If S is a first subtype, then S'Class is a first subtype.
  1. Universal types
      Universal types are defined for (and belong to) the integer, real, and fixed point classes, and are referred to in this standard as respectively, universal_integer, universal_real, and universal_fixed. These are analogous to class-wide types for these language-defined numeric classes. As with class-wide types, if a formal parameter is of a universal type, then an actual parameter of any type in the corresponding class is acceptable. In addition, a value of a universal type (including an integer or real numeric_literal) is "universal" in that it is acceptable where some particular type in the class is expected, see section The Context of Overload Resolution.
    1. The set of values of a universal type is the undiscriminated union of the set of values possible for any definable type in the associated class. Like class-wide types, universal types have no primitive subprograms of their own. However, their "universality" allows them to be used as operands with the primitive subprograms of any type in the corresponding class.
  1. The integer and real numeric classes each have a specific root type in addition to their universal type, named respectively root_integer and root_real.
  2. A class-wide or universal type is said to cover all of the types in its class. A specific type covers only itself.
  3. A specific type T2 is defined to be a descendant of a type T1 if T2 is the same as T1, or if T2 is derived (directly or indirectly) from T1. A class-wide type T2'Class is defined to be a descendant of type T1 if T2 is a descendant of T1. Similarly, the universal types are defined to be descendants of the root types of their classes. If a type T2 is a descendant of a type T1, then T1 is called an ancestor of T2. The ultimate ancestor of a type is the ancestor of the type that is not a descendant of any other type.
  4. An inherited component (including an inherited discriminant) of a derived type is inherited from a given ancestor of the type if the corresponding component was inherited by each derived type in the chain of derivations going back to the given ancestor. NOTES
  5. (18) Because operands of a universal type are acceptable to the predefined operators of any type in their class, ambiguity can result. For universal_integer and universal_real, this potential ambiguity is resolved by giving a preference, see section The Context of Overload Resolution to the predefined operators of the corresponding root types (root_integer and root_real, respectively). Hence, in an apparently ambiguous expression like 
  6. 1 + 4 < 7
  7. where each of the literals is of type universal_integer, the predefined operators of root_integer will be preferred over those of other specific integer types, thereby resolving the ambiguity.

Scalar Types

  1. Scalar types comprise enumeration types, integer types, and real types. Enumeration types and integer types are called discrete types; each value of a discrete type has a position number which is an integer value. Integer types and real types are called numeric types. All scalar types are ordered, that is, all relational operators are predefined for their values.

    Syntax 

  2. range_constraint ::= range range

  3. range ::=
     range_attribute_reference
   | simple_expression .. simple_expression
  4. A range has a lower bound and an upper bound and specifies a subset of the values of some scalar type (the type of the range). A range with lower bound L and upper bound R is described by "L .. R". If R is less than L, then the range is a null range, and specifies an empty set of values. Otherwise, the range specifies the values of the type from the lower bound to the upper bound, inclusive. A value belongs to a range if it is of the type of the range, and is in the subset of values specified by the range. A value satisfies a range constraint if it belongs to the associated range. One range is included in another if all values that belong to the first range also belong to the second.

    Name Resolution Rules

  5. For a subtype_indication containing a range_constraint, either directly or as part of some other scalar_constraint, the type of the range shall resolve to that of the type determined by the subtype_mark of the subtype_indication. For a range of a given type, the simple_expressions of the range (likewise, the simple_expressions of the equivalent range for a range_attribute_reference) are expected to be of the type of the range.

    Static Semantics

  6. The base range of a scalar type is the range of finite values of the type that can be represented in every unconstrained object of the type; it is also the range supported at a minimum for intermediate values during the evaluation of expressions involving predefined operators of the type.
  7. A constrained scalar subtype is one to which a range constraint applies. The range of a constrained scalar subtype is the range associated with the range constraint of the subtype. The range of an unconstrained scalar subtype is the base range of its type.

    Dynamic Semantics

  8. A range is compatible with a scalar subtype if and only if it is either a null range or each bound of the range belongs to the range of the subtype. A range_constraint is compatible with a scalar subtype if and only if its range is compatible with the subtype.
  9. The elaboration of a range_constraint consists of the evaluation of the range. The evaluation of a range determines a lower bound and an upper bound. If simple_expressions are given to specify bounds, the evaluation of the range evaluates these simple_expressions in an arbitrary order, and converts them to the type of the range. If a range_attribute_reference is given, the evaluation of the range consists of the evaluation of the range_attribute_reference.
  10. Attributes
  11. For every scalar subtype S, the following attributes are defined:
  12. S'First 
    S'First denotes the lower bound of the range of S. The value
of this attribute is of the type of S.
  13. S'Last 
    S'Last denotes the upper bound of the range of S. The value
of this attribute is of the type of S.
  14. S'Range 
    S'Range is equivalent to the range S'First .. S'Last.
  15. S'Base 
    S'Base denotes an unconstrained subtype of the type of
S. This unconstrained subtype is called the base subtype of
the type.
  16. S'Min
      S'Min denotes a function with the following specification: 
    1. function S'Min(Left, Right : S'Base)
  return S'Base
    2. The function returns the lesser of the values of the two parameters.
  1. S'Max
      S'Max denotes a function with the following specification: 
    1. function S'Max(Left, Right : S'Base)
  return S'Base
    2. The function returns the greater of the values of the two parameters.
  1. S'Succ
      S'Succ denotes a function with the following specification: 
    1. function S'Succ(Arg : S'Base)
  return S'Base
    2. For an enumeration type, the function returns the value whose position number is one more than that of the value of Arg; Constraint_Error is raised if there is no such value of the type. For an integer type, the function returns the result of adding one to the value of Arg. For a fixed point type, the function returns the result of adding small to the value of Arg. For a floating point type, the function returns the machine number (as defined in section Floating Point Types.) immediately above the value of Arg; Constraint_Error is raised if there is no such machine number.
  1. S'Pred
      S'Pred denotes a function with the following specification: 
    1. function S'Pred(Arg : S'Base)
  return S'Base
    2. For an enumeration type, the function returns the value whose position number is one less than that of the value of Arg; Constraint_Error is raised if there is no such value of the type. For an integer type, the function returns the result of subtracting one from the value of Arg. For a fixed point type, the function returns the result of subtracting small from the value of Arg. For a floating point type, the function returns the machine number (as defined in section Floating Point Types.) immediately below the value of Arg; Constraint_Error is raised if there is no such machine number.
  1. S'Wide_Image
      S'Wide_Image denotes a function with the following specification: 
    1. function S'Wide_Image(Arg : S'Base)
  return Wide_String
    2. The function returns an image of the value of Arg, that is, a sequence of characters representing the value in display form. The lower bound of the result is one.
    3. The image of an integer value is the corresponding decimal literal, without underlines, leading zeros, exponent, or trailing spaces, but with a single leading character that is either a minus sign or a space.
    4. The image of an enumeration value is either the corresponding identifier in upper case or the corresponding character literal (including the two apostrophes); neither leading nor trailing spaces are included. For a nongraphic character (a value of a character type that has no enumeration literal associated with it), the result is a corresponding language-defined or implementation-defined name in upper case (for example, the image of the nongraphic character identified as nul is "NUL" -- the quotes are not part of the image).
    5. The image of a floating point value is a decimal real literal best approximating the value (rounded away from zero if halfway between) with a single leading character that is either a minus sign or a space, a single digit (that is nonzero unless the value is zero), a decimal point, S'Digits-1, see section Operations of Floating Point Types, digits after the decimal point (but one if S'Digits is one), an upper case E, the sign of the exponent (either + or -), and two or more digits (with leading zeros if necessary) representing the exponent. If S'Signed_Zeros is True, then the leading character is a minus sign for a negatively signed zero.
    6. The image of a fixed point value is a decimal real literal best approximating the value (rounded away from zero if halfway between) with a single leading character that is either a minus sign or a space, one or more digits before the decimal point (with no redundant leading zeros), a decimal point, and S'Aft, see section Operations of Fixed Point Types, digits after the decimal point.
  1. S'Image
      S'Image denotes a function with the following specification: 
    1. function S'Image(Arg : S'Base)
  return String
    2. The function returns an image of the value of Arg as a String. The lower bound of the result is one. The image has the same sequence of graphic characters as that defined for S'Wide_Image if all the graphic characters are defined in Character; otherwise the sequence of characters is implementation defined (but no shorter than that of S'Wide_Image for the same value of Arg).
  1. S'Wide_Width 
    S'Wide_Width denotes the maximum length of a Wide_String
returned by S'Wide_Image over all values of the subtype S. It
denotes zero for a subtype that has a null range. Its type
is universal_integer.
  2. S'Width 
    S'Width denotes the maximum length of a String returned by
S'Image over all values of the subtype S. It denotes
zero for a subtype that has a null range. Its type is
universal_integer.
  3. S'Wide_Value
      S'Wide_Value denotes a function with the following specification: 
    1. function S'Wide_Value(Arg : Wide_String)
  return S'Base
    2. This function returns a value given an image of the value as a Wide_String, ignoring any leading or trailing spaces.
    3. For the evaluation of a call on S'Wide_Value for an enumeration subtype S, if the sequence of characters of the parameter (ignoring leading and trailing spaces) has the syntax of an enumeration literal and if it corresponds to a literal of the type of S (or corresponds to the result of S'Wide_Image for a nongraphic character of the type), the result is the corresponding enumeration value; otherwise Constraint_Error is raised.
    4. For the evaluation of a call on S'Wide_Value (or S'Value) for an integer subtype S, if the sequence of characters of the parameter (ignoring leading and trailing spaces) has the syntax of an integer literal, with an optional leading sign character (plus or minus for a signed type; only plus for a modular type), and the corresponding numeric value belongs to the base range of the type of S, then that value is the result; otherwise Constraint_Error is raised.
    5. For the evaluation of a call on S'Wide_Value (or S'Value) for a real subtype S, if the sequence of characters of the parameter (ignoring leading and trailing spaces) has the syntax of one of the following:
      1. numeric_literal
      2. numeral.[exponent]
      3. .numeral[exponent]
      4. base#based_numeral.#[exponent]
      5. base#.based_numeral#[exponent]
    1. with an optional leading sign character (plus or minus), and if the corresponding numeric value belongs to the base range of the type of S, then that value is the result; otherwise Constraint_Error is raised. The sign of a zero value is preserved (positive if none has been specified) if S'Signed_Zeros is True.
  1. S'Value
      S'Value denotes a function with the following specification: 
    1. function S'Value(Arg : String)
  return S'Base
    2. This function returns a value given an image of the value as a String, ignoring any leading or trailing spaces.
    3. For the evaluation of a call on S'Value for an enumeration subtype S, if the sequence of characters of the parameter (ignoring leading and trailing spaces) has the syntax of an enumeration literal and if it corresponds to a literal of the type of S (or corresponds to the result of S'Image for a value of the type), the result is the corresponding enumeration value; otherwise Constraint_Error is raised. For a numeric subtype S, the evaluation of a call on S'Value with Arg of type String is equivalent to a call on S'Wide_Value for a corresponding Arg of type Wide_String.

Implementation Permissions

  1. An implementation may extend the Wide_Value, Value, Wide_Image, and Image attributes of a floating point type to support special values such as infinities and NaNs. NOTES
  2. (19) The evaluation of S'First or S'Last never raises an exception. If a scalar subtype S has a nonnull range, S'First and S'Last belong to this range. These values can, for example, always be assigned to a variable of subtype S.
  3. (20) For a subtype of a scalar type, the result delivered by the attributes Succ, Pred, and Value might not belong to the subtype; similarly, the actual parameters of the attributes Succ, Pred, and Image need not belong to the subtype.
  4. (21) For any value V (including any nongraphic character) of an enumeration subtype S, S'Value(S'Image(V)) equals V, as does S'Wide_Value(S'Wide_Image(V)). Neither expression ever raises Constraint_Error.

    Examples

  5. Examples of ranges: 
  6. -10 .. 10
X .. X + 1
0.0 .. 2.0*Pi
Red .. Green     --  see section Enumeration Types
1 .. 0           -- a null range
Table'Range      -- a range attribute reference, see section Array Types
  7. Examples of range constraints: 
  8. range -999.0 .. +999.0
range S'First+1 .. S'Last-1

Enumeration Types

  1. An enumeration_type_definition defines an enumeration type.

    Syntax 

  2. enumeration_type_definition ::=
   (enumeration_literal_specification
     {, enumeration_literal_specification})

  3. enumeration_literal_specification ::=
   defining_identifier | defining_character_literal

  4. defining_character_literal ::= character_literal

    Legality Rules

  5. The defining_identifiers and defining_character_literals listed in an enumeration_type_definition shall be distinct.

    Static Semantics

  6. Each enumeration_literal_specification is the explicit declaration of the corresponding enumeration literal: it declares a parameterless function, whose defining name is the defining_identifier or defining_character_literal, and whose result type is the enumeration type.
  7. Each enumeration literal corresponds to a distinct value of the enumeration type, and to a distinct position number. The position number of the value of the first listed enumeration literal is zero; the position number of the value of each subsequent enumeration literal is one more than that of its predecessor in the list.
  8. The predefined order relations between values of the enumeration type follow the order of corresponding position numbers.
  9. If the same defining_identifier or defining_character_literal is specified in more than one enumeration_type_definition, the corresponding enumeration literals are said to be overloaded. At any place where an overloaded enumeration literal occurs in the text of a program, the type of the enumeration literal has to be determinable from the context, see section The Context of Overload Resolution.

    Dynamic Semantics

  10. The elaboration of an enumeration_type_definition creates the enumeration type and its first subtype, which is constrained to the base range of the type.
  11. When called, the parameterless function associated with an enumeration literal returns the corresponding value of the enumeration type. NOTES
  12. (22) If an enumeration literal occurs in a context that does not otherwise suffice to determine the type of the literal, then qualification by the name of the enumeration type is one way to resolve the ambiguity, see section Qualified Expressions.

    Examples

  13. Examples of enumeration types and subtypes: 
  14. type Day    is (Mon, Tue, Wed, Thu, Fri, Sat, Sun);
type Suit   is (Clubs, Diamonds, Hearts, Spades);
type Gender is (M, F);
type Level  is (Low, Medium, Urgent);
type Color  is (White, Red, Yellow, Green, Blue, Brown, Black);
type Light  is (Red, Amber, Green); -- Red and Green are overloaded

  15. type Hexa   is ('A', 'B', 'C', 'D', 'E', 'F');
type Mixed  is ('A', 'B', '*', B, None, '?', '%');

  16. subtype Weekday is Day   range Mon .. Fri;
subtype Major   is Suit  range Hearts .. Spades;
subtype Rainbow is Color range Red .. Blue;
--  the Color Red, not the Light

Character Types

Static Semantics

  1. An enumeration type is said to be a character type if at least one of its enumeration literals is a character_literal.
  2. The predefined type Character is a character type whose values correspond to the 256 code positions of Row 00 (also known as Latin-1) of the ISO 10646 Basic Multilingual Plane (BMP). Each of the graphic characters of Row 00 of the BMP has a corresponding character_literal in Character. Each of the nongraphic positions of Row 00 (0000-001F and 007F-009F) has a corresponding language-defined name, which is not usable as an enumeration literal, but which is usable with the attributes (Wide_)Image and (Wide_)Value; these names are given in the definition of type Character in section The Package Standard, but are set in italics.
  3. The predefined type Wide_Character is a character type whose values correspond to the 65536 code positions of the ISO 10646 Basic Multilingual Plane (BMP). Each of the graphic characters of the BMP has a corresponding character_literal in Wide_Character. The first 256 values of Wide_Character have the same character_literal or language-defined name as defined for Character. The last 2 values of Wide_Character correspond to the nongraphic positions FFFE and FFFF of the BMP, and are assigned the language-defined names FFFE and FFFF. As with the other language-defined names for nongraphic characters, the names FFFE and FFFF are usable only with the attributes (Wide_)Image and (Wide_)Value; they are not usable as enumeration literals. All other values of Wide_Character are considered graphic characters, and have a corresponding character_literal.

    Implementation Permissions

  4. In a nonstandard mode, an implementation may provide other interpretations for the predefined types Character and Wide_Character, to conform to local conventions.

    Implementation Advice

  5. If an implementation supports a mode with alternative interpretations for Character and Wide_Character, the set of graphic characters of Character should nevertheless remain a proper subset of the set of graphic characters of Wide_Character. Any character set "localizations" should be reflected in the results of the subprograms defined in the language-defined package Characters.Handling, see section Character Handling, available in such a mode. In a mode with an alternative interpretation of Character, the implementation should also support a corresponding change in what is a legal identifier_letter. NOTES
  6. (23) The language-defined library package Characters.Latin_1 (section The Package Characters.Latin_1) includes the declaration of constants denoting control characters, lower case characters, and special characters of the predefined type Character.
  7. (24) A conventional character set such as EBCDIC can be declared as a character type; the internal codes of the characters can be specified by an enumeration_representation_clause as explained in clause (see section Enumeration Representation Clauses).

    Examples

  8. Example of a character type: 
  9. type Roman_Digit is ('I', 'V', 'X', 'L', 'C', 'D', 'M');

Boolean Types

Static Semantics

  1. There is a predefined enumeration type named Boolean, declared in the visible part of package Standard. It has the two enumeration literals False and True ordered with the relation False < True. Any descendant of the predefined type Boolean is called a boolean type.

Integer Types

  1. An integer_type_definition defines an integer type; it defines either a signed integer type, or a modular integer type. The base range of a signed integer type includes at least the values of the specified range. A modular type is an integer type with all arithmetic modulo a specified positive modulus; such a type corresponds to an unsigned type with wrap-around semantics.

    Syntax 

  2. integer_type_definition ::=
   signed_integer_type_definition | modular_type_definition

  3. signed_integer_type_definition ::=
   range static_simple_expression .. static_simple_expression

  4. modular_type_definition ::= mod static_expression

    Name Resolution Rules

  5. Each simple_expression in a signed_integer_type_definition is expected to be of any integer type; they need not be of the same type. The expression in a modular_type_definition is likewise expected to be of any integer type.

    Legality Rules

  6. The simple_expressions of a signed_integer_type_definition shall be static, and their values shall be in the range System.Min_Int .. System.Max_Int.
  7. The expression of a modular_type_definition shall be static, and its value (the modulus) shall be positive, and shall be no greater than System.Max_Binary_Modulus if a power of 2, or no greater than System.Max_Nonbinary_Modulus if not.

    Static Semantics

  8. The set of values for a signed integer type is the (infinite) set of mathematical integers, though only values of the base range of the type are fully supported for run-time operations. The set of values for a modular integer type are the values from 0 to one less than the modulus, inclusive.
  9. A signed_integer_type_definition defines an integer type whose base range includes at least the values of the simple_expressions and is symmetric about zero, excepting possibly an extra negative value. A signed_integer_type_definition also defines a constrained first subtype of the type, with a range whose bounds are given by the values of the simple_expressions, converted to the type being defined.
  10. A modular_type_definition defines a modular type whose base range is from zero to one less than the given modulus. A modular_type_definition also defines a constrained first subtype of the type with a range that is the same as the base range of the type.
  11. There is a predefined signed integer subtype named Integer, declared in the visible part of package Standard. It is constrained to the base range of its type.
  12. Integer has two predefined subtypes, declared in the visible part of package Standard: 
  13. subtype Natural  is Integer range 0 .. Integer'Last;
subtype Positive is Integer range 1 .. Integer'Last;
  14. A type defined by an integer_type_definition is implicitly derived from root_integer, an anonymous predefined (specific) integer type, whose base range is System.Min_Int .. System.Max_Int. However, the base range of the new type is not inherited from root_integer, but is instead determined by the range or modulus specified by the integer_type_definition. Integer literals are all of the type universal_integer, the universal type, see section Derivation Classes, for the class rooted at root_integer, allowing their use with the operations of any integer type.
  15. The position number of an integer value is equal to the value.
  16. For every modular subtype S, the following attribute is defined:
  17. S'Modulus 
    S'Modulus yields the modulus of the type of S, as a value of
the type universal_integer.

    Dynamic Semantics

  18. The elaboration of an integer_type_definition creates the integer type and its first subtype.
  19. For a modular type, if the result of the execution of a predefined operator, see section Operators and Expression Evaluation is outside the base range of the type, the result is reduced modulo the modulus of the type to a value that is within the base range of the type.
  20. For a signed integer type, the exception Constraint_Error is raised by the execution of an operation that cannot deliver the correct result because it is outside the base range of the type. For any integer type, Constraint_Error is raised by the operators "/", "rem", and "mod" if the right operand is zero.

    Implementation Requirements

  21. In an implementation, the range of Integer shall include the range -2**15+1 .. +2**15-1.
  22. If Long_Integer is predefined for an implementation, then its range shall include the range -2**31+1 .. +2**31-1.
  23. System.Max_Binary_Modulus shall be at least 2**16.

    Implementation Permissions

  24. For the execution of a predefined operation of a signed integer type, the implementation need not raise Constraint_Error if the result is outside the base range of the type, so long as the correct result is produced.
  25. An implementation may provide additional predefined signed integer types, declared in the visible part of Standard, whose first subtypes have names of the form Short_Integer, Long_Integer, Short_Short_Integer, Long_Long_Integer, etc. Different predefined integer types are allowed to have the same base range. However, the range of Integer should be no wider than that of Long_Integer. Similarly, the range of Short_Integer (if provided) should be no wider than Integer. Corresponding recommendations apply to any other predefined integer types. There need not be a named integer type corresponding to each distinct base range supported by an implementation. The range of each first subtype should be the base range of its type.
  26. An implementation may provide nonstandard integer types, descendants of root_integer that are declared outside of the specification of package Standard, which need not have all the standard characteristics of a type defined by an integer_type_definition. For example, a nonstandard integer type might have an asymmetric base range or it might not be allowed as an array or loop index (a very long integer). Any type descended from a nonstandard integer type is also nonstandard. An implementation may place arbitrary restrictions on the use of such types; it is implementation defined whether operators that are predefined for "any integer type" are defined for a particular nonstandard integer type. In any case, such types are not permitted as explicit_generic_actual_parameters for formal scalar types -- see section Formal Scalar Types.
  27. For a one's complement machine, the high bound of the base range of a modular type whose modulus is one less than a power of 2 may be equal to the modulus, rather than one less than the modulus. It is implementation defined for which powers of 2, if any, this permission is exercised.

    Implementation Advice

  28. An implementation should support Long_Integer in addition to Integer if the target machine supports 32-bit (or longer) arithmetic. No other named integer subtypes are recommended for package Standard. Instead, appropriate named integer subtypes should be provided in the library package Interfaces, see section The Package Interfaces.
  29. An implementation for a two's complement machine should support modular types with a binary modulus up to System.Max_Int*2+2. An implementation should support a nonbinary modulus up to Integer'Last. NOTES
  30. (25) Integer literals are of the anonymous predefined integer type universal_integer. Other integer types have no literals. However, the overload resolution rules, see section The Context of Overload Resolution, allow expressions of the type universal_integer whenever an integer type is expected.
  31. (26) The same arithmetic operators are predefined for all signed integer types defined by a signed_integer_type_definition, see section Operators and Expression Evaluation. For modular types, these same operators are predefined, plus bit-wise logical operators (and, or, xor, and not). In addition, for the unsigned types declared in the language-defined package Interfaces, see section The Package Interfaces, functions are defined that provide bit-wise shifting and rotating.
  32. (27) Modular types match a generic_formal_parameter_declaration of the form "type T is mod <>;"; signed integer types match "type T is range <>;", see section Formal Scalar Types.

    Examples

  33. Examples of integer types and subtypes: 
  34. type Page_Num  is range 1 .. 2_000;
type Line_Size is range 1 .. Max_Line_Size;

  35. subtype Small_Int   is Integer   range -10 .. 10;
subtype Column_Ptr  is Line_Size range 1 .. 10;
subtype Buffer_Size is Integer   range 0 .. Max;

  36. type Byte        is mod 256; -- an unsigned byte
type Hash_Index  is mod 97;  -- modulus is prime

Operations of Discrete Types

Static Semantics

  1. For every discrete subtype S, the following attributes are defined:
  2. S'Pos
      S'Pos denotes a function with the following specification: 
    1. function S'Pos(Arg : S'Base)
  return universal_integer
    2. This function returns the position number of the value of Arg, as a value of type universal_integer.
  1. S'Val
      S'Val denotes a function with the following specification: 
    1. function S'Val(Arg : universal_integer)
  return S'Base
    2. This function returns a value of the type of S whose position number equals the value of Arg. For the evaluation of a call on S'Val, if there is no value in the base range of its type with the given position number, Constraint_Error is raised.

Implementation Advice

  1. For the evaluation of a call on S'Pos for an enumeration subtype, if the value of the operand does not correspond to the internal code for any enumeration literal of its type (perhaps due to an uninitialized variable), then the implementation should raise Program_Error. This is particularly important for enumeration types with noncontiguous internal codes specified by an enumeration_representation_clause. NOTES
  2. (28) Indexing and loop iteration use values of discrete types.
  3. (29) The predefined operations of a discrete type include the assignment operation, qualification, the membership tests, and the relational operators; for a boolean type they include the short-circuit control forms and the logical operators; for an integer type they include type conversion to and from other numeric types, as well as the binary and unary adding operators - and +, the multiplying operators, the unary operator abs, and the exponentiation operator. The assignment operation is described in section Assignment Statements. The other predefined operations are described in Section section Names and Expressions.
  4. (30) As for all types, objects of a discrete type have Size and Address attributes, see section Representation Attributes.
  5. (31) For a subtype of a discrete type, the result delivered by the attribute Val might not belong to the subtype; similarly, the actual parameter of the attribute Pos need not belong to the subtype. The following relations are satisfied (in the absence of an exception) by these attributes: 
  6. S'Val(S'Pos(X)) = X
S'Pos(S'Val(N)) = N

    Examples

  7. Examples of attributes of discrete subtypes: 
  8. --  For the types and subtypes declared in subclause
--  see section Enumeration Types, the following hold:

  9. --  Color'First   = White,   Color'Last   = Black
--  Rainbow'First = Red,     Rainbow'Last = Blue

  10. --  Color'Succ(Blue) = Rainbow'Succ(Blue) = Brown
--  Color'Pos(Blue)  = Rainbow'Pos(Blue)  = 4
--  Color'Val(0)     = Rainbow'Val(0)     = White

Real Types

  1. Real types provide approximations to the real numbers, with relative bounds on errors for floating point types, and with absolute bounds for fixed point types.

    Syntax 

  2. real_type_definition ::=
   floating_point_definition | fixed_point_definition

    Static Semantics

  3. A type defined by a real_type_definition is implicitly derived from root_real, an anonymous predefined (specific) real type. Hence, all real types, whether floating point or fixed point, are in the derivation class rooted at root_real.
  4. Real literals are all of the type universal_real, the universal type (see section Derivation Classes) for the class rooted at root_real, allowing their use with the operations of any real type. Certain multiplying operators have a result type of universal_fixed, see section Multiplying Operators, the universal type for the class of fixed point types, allowing the result of the multiplication or division to be used where any specific fixed point type is expected.

    Dynamic Semantics

  5. The elaboration of a real_type_definition consists of the elaboration of the floating_point_definition or the fixed_point_definition.

    Implementation Requirements

  6. An implementation shall perform the run-time evaluation of a use of a predefined operator of root_real with an accuracy at least as great as that of any floating point type definable by a floating_point_definition.

    Implementation Permissions

  7. For the execution of a predefined operation of a real type, the implementation need not raise Constraint_Error if the result is outside the base range of the type, so long as the correct result is produced, or the Machine_Overflows attribute of the type is false, see section Numeric Performance Requirements.
  8. An implementation may provide nonstandard real types, descendants of root_real that are declared outside of the specification of package Standard, which need not have all the standard characteristics of a type defined by a real_type_definition. For example, a nonstandard real type might have an asymmetric or unsigned base range, or its predefined operations might wrap around or "saturate" rather than overflow (modular or saturating arithmetic), or it might not conform to the accuracy model, see section Numeric Performance Requirements. Any type descended from a nonstandard real type is also nonstandard. An implementation may place arbitrary restrictions on the use of such types; it is implementation defined whether operators that are predefined for "any real type" are defined for a particular nonstandard real type. In any case, such types are not permitted as explicit_generic_actual_parameters for formal scalar types -- see section Formal Scalar Types. NOTES
  9. (32) As stated, real literals are of the anonymous predefined real type universal_real. Other real types have no literals. However, the overload resolution rules, see section The Context of Overload Resolution, allow expressions of the type universal_real whenever a real type is expected.

Floating Point Types

  1. For floating point types, the error bound is specified as a relative precision by giving the required minimum number of significant decimal digits.

    Syntax 

  2. floating_point_definition ::=
   digits static_expression [real_range_specification]

  3. real_range_specification ::=
   range static_simple_expression .. static_simple_expression

    Name Resolution Rules

  4. The requested decimal precision, which is the minimum number of significant decimal digits required for the floating point type, is specified by the value of the expression given after the reserved word digits. This expression is expected to be of any integer type.
  5. Each simple_expression of a real_range_specification is expected to be of any real type; the types need not be the same.

    Legality Rules

  6. The requested decimal precision shall be specified by a static expression whose value is positive and no greater than System.Max_Base_Digits. Each simple_expression of a real_range_specification shall also be static. If the real_range_specification is omitted, the requested decimal precision shall be no greater than System.Max_Digits.
  7. A floating_point_definition is illegal if the implementation does not support a floating point type that satisfies the requested decimal precision and range.

    Static Semantics

  8. The set of values for a floating point type is the (infinite) set of rational numbers. The machine numbers of a floating point type are the values of the type that can be represented exactly in every unconstrained variable of the type. The base range, see section Scalar Types, of a floating point type is symmetric around zero, except that it can include some extra negative values in some implementations.
  9. The base decimal precision of a floating point type is the number of decimal digits of precision representable in objects of the type. The safe range of a floating point type is that part of its base range for which the accuracy corresponding to the base decimal precision is preserved by all predefined operations.
  10. A floating_point_definition defines a floating point type whose base decimal precision is no less than the requested decimal precision. If a real_range_specification is given, the safe range of the floating point type (and hence, also its base range) includes at least the values of the simple expressions given in the real_range_specification. If a real_range_specification is not given, the safe (and base) range of the type includes at least the values of the range -10.0**(4*D) .. +10.0**(4*D) where D is the requested decimal precision. The safe range might include other values as well. The attributes Safe_First and Safe_Last give the actual bounds of the safe range.
  11. A floating_point_definition also defines a first subtype of the type. If a real_range_specification is given, then the subtype is constrained to a range whose bounds are given by a conversion of the values of the simple_expressions of the real_range_specification to the type being defined. Otherwise, the subtype is unconstrained.
  12. There is a predefined, unconstrained, floating point subtype named Float, declared in the visible part of package Standard.

    Dynamic Semantics

  13. The elaboration of a floating_point_definition creates the floating point type and its first subtype.

    Implementation Requirements

  14. In an implementation that supports floating point types with 6 or more digits of precision, the requested decimal precision for Float shall be at least 6.
  15. If Long_Float is predefined for an implementation, then its requested decimal precision shall be at least 11.

    Implementation Permissions

  16. An implementation is allowed to provide additional predefined floating point types, declared in the visible part of Standard, whose (unconstrained) first subtypes have names of the form Short_Float, Long_Float, Short_Short_Float, Long_Long_Float, etc. Different predefined floating point types are allowed to have the same base decimal precision. However, the precision of Float should be no greater than that of Long_Float. Similarly, the precision of Short_Float (if provided) should be no greater than Float. Corresponding recommendations apply to any other predefined floating point types. There need not be a named floating point type corresponding to each distinct base decimal precision supported by an implementation.

    Implementation Advice

  17. An implementation should support Long_Float in addition to Float if the target machine supports 11 or more digits of precision. No other named floating point subtypes are recommended for package Standard. Instead, appropriate named floating point subtypes should be provided in the library package Interfaces, see section The Package Interfaces. NOTES
  18. (33) If a floating point subtype is unconstrained, then assignments to variables of the subtype involve only Overflow_Checks, never Range_Checks.

    Examples

  19. Examples of floating point types and subtypes: 
  20. type Coefficient is digits 10 range -1.0 .. 1.0;

  21. type Real is digits 8;
type Mass is digits 7 range 0.0 .. 1.0E35;

  22. subtype Probability is Real range 0.0 .. 1.0;
--  a subtype with a smaller range

Operations of Floating Point Types

Static Semantics

  1. The following attribute is defined for every floating point subtype S:
  2. S'Digits 
    S'Digits denotes the requested decimal precision for the
subtype S. The value of this attribute is of the type
universal_integer. The requested decimal precision of the
base subtype of a floating point type T is defined to be the
largest value of d for which ceiling(d * log(10) /
log(T'Machine_Radix)) + 1 <= T'Model_Mantissa.
    NOTES
  3. (34) The predefined operations of a floating point type include the assignment operation, qualification, the membership tests, and explicit conversion to and from other numeric types. They also include the relational operators and the following predefined arithmetic operators: the binary and unary adding operators - and +, certain multiplying operators, the unary operator abs, and the exponentiation operator.
  4. (35) As for all types, objects of a floating point type have Size and Address attributes, see section Representation Attributes. Other attributes of floating point types are defined in section Attributes of Floating Point Types.

Fixed Point Types

  1. A fixed point type is either an ordinary fixed point type, or a decimal fixed point type. The error bound of a fixed point type is specified as an absolute value, called the delta of the fixed point type.

    Syntax 

  2. fixed_point_definition ::=
   ordinary_fixed_point_definition | decimal_fixed_point_definition

  3. ordinary_fixed_point_definition ::=
   delta static_expression real_range_specification

  4. decimal_fixed_point_definition ::=
   delta static_expression digits static_expression
     [real_range_specification]

  5. digits_constraint ::=
   digits static_expression [range_constraint]

    Name Resolution Rules

  6. For a type defined by a fixed_point_definition, the delta of the type is specified by the value of the expression given after the reserved word delta; this expression is expected to be of any real type. For a type defined by a decimal_fixed_point_definition (a decimal fixed point type), the number of significant decimal digits for its first subtype (the digits of the first subtype) is specified by the expression given after the reserved word digits; this expression is expected to be of any integer type.

    Legality Rules

  7. In a fixed_point_definition or digits_constraint, the expressions given after the reserved words delta and digits shall be static; their values shall be positive.
  8. The set of values of a fixed point type comprise the integral multiples of a number called the small of the type. For a type defined by an ordinary_fixed_point_definition (an ordinary fixed point type), the small may be specified by an attribute_definition_clause, see section Representation Attributes, if so specified, it shall be no greater than the delta of the type. If not specified, the small of an ordinary fixed point type is an implementation-defined power of two less than or equal to the delta.
  9. For a decimal fixed point type, the small equals the delta; the delta shall be a power of 10. If a real_range_specification is given, both bounds of the range shall be in the range -(10**digits-1)*delta .. +(10**digits-1)*delta.
  10. A fixed_point_definition is illegal if the implementation does not support a fixed point type with the given small and specified range or digits.
  11. For a subtype_indication with a digits_constraint, the subtype_mark shall denote a decimal fixed point subtype.

    Static Semantics

  12. The base range, see section Scalar Types of a fixed point type is symmetric around zero, except possibly for an extra negative value in some implementations.
  13. An ordinary_fixed_point_definition defines an ordinary fixed point type whose base range includes at least all multiples of small that are between the bounds specified in the real_range_specification. The base range of the type does not necessarily include the specified bounds themselves. An ordinary_fixed_point_definition also defines a constrained first subtype of the type, with each bound of its range given by the closer to zero of:
    1. the value of the conversion to the fixed point type of the corresponding expression of the real_range_specification;
    2. the corresponding bound of the base range.
  1. A decimal_fixed_point_definition defines a decimal fixed point type whose base range includes at least the range -(10**digits-1)*delta .. +(10**digits-1)*delta. A decimal_fixed_point_definition also defines a constrained first subtype of the type. If a real_range_specification is given, the bounds of the first subtype are given by a conversion of the values of the expressions of the real_range_specification. Otherwise, the range of the first subtype is -(10**digits-1)*delta .. +(10**digits-1)*delta.

    Dynamic Semantics

  2. The elaboration of a fixed_point_definition creates the fixed point type and its first subtype.
  3. For a digits_constraint on a decimal fixed point subtype with a given delta, if it does not have a range_constraint, then it specifies an implicit range -(10**D-1)*delta .. +(10**D-1)*delta, where D is the value of the expression. A digits_constraint is compatible with a decimal fixed point subtype if the value of the expression is no greater than the digits of the subtype, and if it specifies (explicitly or implicitly) a range that is compatible with the subtype.
  4. The elaboration of a digits_constraint consists of the elaboration of the range_constraint, if any. If a range_constraint is given, a check is made that the bounds of the range are both in the range -(10**D-1)*delta .. +(10**D-1)*delta, where D is the value of the (static) expression given after the reserved word digits. If this check fails, Constraint_Error is raised.

    Implementation Requirements

  5. The implementation shall support at least 24 bits of precision (including the sign bit) for fixed point types.

    Implementation Permissions

  6. Implementations are permitted to support only smalls that are a power of two. In particular, all decimal fixed point type declarations can be disallowed. Note however that conformance with the Information Systems Annex requires support for decimal smalls, and decimal fixed point type declarations with digits up to at least 18. NOTES
  7. (36) The base range of an ordinary fixed point type need not include the specified bounds themselves so that the range specification can be given in a natural way, such as: 
  8. type Fraction is delta 2.0**(-15) range -1.0 .. 1.0;
  9. With 2's complement hardware, such a type could have a signed 16-bit representation, using 1 bit for the sign and 15 bits for fraction, resulting in a base range of -1.0 .. 1.0-2.0**(-15).

    Examples

  10. Examples of fixed point types and subtypes: 
  11. type Volt is delta 0.125 range 0.0 .. 255.0;

  12. --  A pure fraction which requires all the available
--  space in a word can be declared as the type Fraction:

type Fraction is delta System.Fine_Delta range -1.0 .. 1.0;
--  Fraction'Last = 1.0 - System.Fine_Delta

  13. type Money is delta 0.01 digits 15;  -- decimal fixed point
subtype Salary is Money digits 10;
  -- Money'Last = 10.0**13 - 0.01, Salary'Last = 10.0**8 - 0.01

Operations of Fixed Point Types

Static Semantics

  1. The following attributes are defined for every fixed point subtype S:
  2. S'Small 
    S'Small denotes the small of the type of S. The value of
this attribute is of the type universal_real. Small may
be specified for nonderived fixed point types via an
attribute_definition_clause, see section Representation Attributes; the expression
of such a clause shall be static.
  3. S'Delta 
    S'Delta denotes the delta of the fixed point subtype S. The
value of this attribute is of the type universal_real.
  4. S'Fore 
    S'Fore yields the minimum number of characters needed
before the decimal point for the decimal representation
of any value of the subtype S, assuming that the
representation does not include an exponent, but
includes a one-character prefix that is either a minus
sign or a space. (This minimum number does not include
superfluous zeros or underlines, and is at least 2.) The
value of this attribute is of the type
universal_integer.
  5. S'Aft 
    S'Aft yields the number of decimal digits needed after the
decimal point to accommodate the delta of the subtype S,
unless the delta of the subtype S is greater than 0.1, in
which case the attribute yields the value one. (S'Aft is the
smallest positive integer N for which (10**N)*S'Delta is
greater than or equal to one.) The value of this attribute
is of the type universal_integer.
  6. The following additional attributes are defined for every decimal fixed point subtype S:
  7. S'Digits 
    S'Digits denotes the digits of the decimal fixed point
subtype S, which corresponds to the number of decimal digits
that are representable in objects of the subtype. The value
of this attribute is of the type universal_integer. Its
value is determined as follows:
    1. For a first subtype or a subtype defined by a subtype_indication with a digits_constraint, the digits is the value of the expression given after the reserved word digits;
    2. For a subtype defined by a subtype_indication without a digits_constraint, the digits of the subtype is the same as that of the subtype denoted by the subtype_mark in the subtype_indication.
    3. The digits of a base subtype is the largest integer D such that the range -(10**D-1)*delta .. +(10**D-1)*delta is included in the base range of the type.
  1. S'Scale 
    S'Scale denotes the scale of the subtype S, defined as the
value N such that S'Delta = 10.0**(-N). The scale indicates
the position of the point relative to the rightmost
significant digits of values of subtype S. The value of this
attribute is of the type universal_integer.
  2. S'Round
      S'Round denotes a function with the following specification: 
    1. function S'Round(X : universal_real)
  return S'Base
    2. The function returns the value obtained by rounding X (away from 0, if X is midway between two values of the type of S).
    NOTES
  1. (37) All subtypes of a fixed point type will have the same value for the Delta attribute, in the absence of delta_constraints, see section Reduced Accuracy Subtypes.
  2. (38) S'Scale is not always the same as S'Aft for a decimal subtype; for example, if S'Delta = 1.0 then S'Aft is 1 while S'Scale is 0.
  3. (39) The predefined operations of a fixed point type include the assignment operation, qualification, the membership tests, and explicit conversion to and from other numeric types. They also include the relational operators and the following predefined arithmetic operators: the binary and unary adding operators - and +, multiplying operators, and the unary operator abs.
  4. (40) As for all types, objects of a fixed point type have Size and Address attributes, see section Representation Attributes. Other attributes of fixed point types are defined in section Attributes of Fixed Point Types.

Array Types

  1. An array object is a composite object consisting of components which all have the same subtype. The name for a component of an array uses one or more index values belonging to specified discrete types. The value of an array object is a composite value consisting of the values of the components.

    Syntax 

  2. array_type_definition ::=
   unconstrained_array_definition | constrained_array_definition

  3. unconstrained_array_definition ::=
   array(index_subtype_definition {, index_subtype_definition})
     of component_definition

  4. index_subtype_definition ::= subtype_mark range <>

  5. constrained_array_definition ::=
   array (discrete_subtype_definition
     {, discrete_subtype_definition}) of component_definition

  6. discrete_subtype_definition ::= discrete_subtype_indication | range

  7. component_definition ::= [aliased] subtype_indication

    Name Resolution Rules

  8. For a discrete_subtype_definition that is a range, the range shall resolve to be of some specific discrete type; which discrete type shall be determined without using any context other than the bounds of the range itself (plus the preference for root_integer -- see section The Context of Overload Resolution.).

    Legality Rules

  9. Each index_subtype_definition or discrete_subtype_definition in an array_type_definition defines an index subtype; its type (the index type) shall be discrete.
  10. The subtype defined by the subtype_indication of a component_definition (the component subtype) shall be a definite subtype.
  11. Within the definition of a nonlimited composite type (or a limited composite type that later in its immediate scope becomes nonlimited -- section Private Operations, and section Limited Types.), if a component_definition contains the reserved word aliased and the type of the component is discriminated, then the nominal subtype of the component shall be constrained.

    Static Semantics

  12. An array is characterized by the number of indices (the dimensionality of the array), the type and position of each index, the lower and upper bounds for each index, and the subtype of the components. The order of the indices is significant.
  13. A one-dimensional array has a distinct component for each possible index value. A multidimensional array has a distinct component for each possible sequence of index values that can be formed by selecting one value for each index position (in the given order). The possible values for a given index are all the values between the lower and upper bounds, inclusive; this range of values is called the index range. The bounds of an array are the bounds of its index ranges. The length of a dimension of an array is the number of values of the index range of the dimension (zero for a null range). The length of a one-dimensional array is the length of its only dimension.
  14. An array_type_definition defines an array type and its first subtype. For each object of this array type, the number of indices, the type and position of each index, and the subtype of the components are as in the type definition; the values of the lower and upper bounds for each index belong to the corresponding index subtype of its type, except for null arrays, see section Index Constraints and Discrete Ranges.
  15. An unconstrained_array_definition defines an array type with an unconstrained first subtype. Each index_subtype_definition defines the corresponding index subtype to be the subtype denoted by the subtype_mark. The compound delimiter <> (called a box) of an index_subtype_definition stands for an undefined range (different objects of the type need not have the same bounds).
  16. A constrained_array_definition defines an array type with a constrained first subtype. Each discrete_subtype_definition defines the corresponding index subtype, as well as the corresponding index range for the constrained first subtype. The constraint of the first subtype consists of the bounds of the index ranges.
  17. The discrete subtype defined by a discrete_subtype_definition is either that defined by the subtype_indication, or a subtype determined by the range as follows:
    1. If the type of the range resolves to root_integer, then the discrete_subtype_definition defines a subtype of the predefined type Integer with bounds given by a conversion to Integer of the bounds of the range;
    2. Otherwise, the discrete_subtype_definition defines a subtype of the type of the range, with the bounds given by the range.
  1. The component_definition of an array_type_definition defines the nominal subtype of the components. If the reserved word aliased appears in the component_definition, then each component of the array is aliased, see section Access Types.

    Dynamic Semantics

  2. The elaboration of an array_type_definition creates the array type and its first subtype, and consists of the elaboration of any discrete_subtype_definitions and the component_definition.
  3. The elaboration of a discrete_subtype_definition creates the discrete subtype, and consists of the elaboration of the subtype_indication or the evaluation of the range. The elaboration of a component_definition in an array_type_definition consists of the elaboration of the subtype_indication. The elaboration of any discrete_subtype_definitions and the elaboration of the component_definition are performed in an arbitrary order. NOTES
  4. (41) All components of an array have the same subtype. In particular, for an array of components that are one-dimensional arrays, this means that all components have the same bounds and hence the same length.
  5. (42) Each elaboration of an array_type_definition creates a distinct array type. A consequence of this is that each object whose object_declaration contains an array_type_definition is of its own unique type.

    Examples

  6. Examples of type declarations with unconstrained array definitions: 
  7. type Vector     is array(Integer  range <>) of Real;

type Matrix     is array(Integer  range <>, Integer range <>)
  of Real;

type Bit_Vector is array(Integer  range <>) of Boolean;

type Roman      is array(Positive range <>) of
  Roman_Digit;  --  see section Character Types
  8. Examples of type declarations with constrained array definitions: 
  9. type Table    is array(1 .. 10) of Integer;
type Schedule is array(Day) of Boolean;
type Line     is array(1 .. Max_Line_Size) of Character;
  10. Examples of object declarations with array type definitions: 
  11. Grid : array(1 .. 80, 1 .. 100) of Boolean;
Mix  : array(Color range Red .. Green) of Boolean;
Page : array(Positive range <>) of Line :=  --  an array of arrays
  (1 | 50  => Line'(1 | Line'Last => '+', others => '-'),
  --  see section Array Aggregates
   2 .. 49 => Line'(1 | Line'Last => '|', others => ' '));
    -- Page is constrained by its initial value to (1..50)

Index Constraints and Discrete Ranges

  1. An index_constraint determines the range of possible values for every index of an array subtype, and thereby the corresponding array bounds.

    Syntax 

  2. index_constraint ::= (discrete_range {, discrete_range})

  3. discrete_range ::= discrete_subtype_indication | range

    Name Resolution Rules

  4. The type of a discrete_range is the type of the subtype defined by the subtype_indication, or the type of the range. For an index_constraint, each discrete_range shall resolve to be of the type of the corresponding index.

    Legality Rules

  5. An index_constraint shall appear only in a subtype_indication whose subtype_mark denotes either an unconstrained array subtype, or an unconstrained access subtype whose designated subtype is an unconstrained array subtype; in either case, the index_constraint shall provide a discrete_range for each index of the array type.

    Static Semantics

  6. A discrete_range defines a range whose bounds are given by the range, or by the range of the subtype defined by the subtype_indication.

    Dynamic Semantics

  7. An index_constraint is compatible with an unconstrained array subtype if and only if the index range defined by each discrete_range is compatible, see section Scalar Types, with the corresponding index subtype. If any of the discrete_ranges defines a null range, any array thus constrained is a null array, having no components. An array value satisfies an index_constraint if at each index position the array value and the index_constraint have the same index bounds.
  8. The elaboration of an index_constraint consists of the evaluation of the discrete_range(s), in an arbitrary order. The evaluation of a discrete_range consists of the elaboration of the subtype_indication or the evaluation of the range. NOTES
  9. (43) The elaboration of a subtype_indication consisting of a subtype_mark followed by an index_constraint checks the compatibility of the index_constraint with the subtype_mark, see section Subtype Declarations.
  10. (44) Even if an array value does not satisfy the index constraint of an array subtype, Constraint_Error is not raised on conversion to the array subtype, so long as the length of each dimension of the array value and the array subtype match (see section Type Conversions).

    Examples

  11. Examples of array declarations including an index constraint: 
  12. Board     : Matrix(1 .. 8,  1 .. 8);  --  see section Array Types
Rectangle : Matrix(1 .. 20, 1 .. 30);
Inverse   : Matrix(1 .. N,  1 .. N);  --  N need not be static

  13. Filter    : Bit_Vector(0 .. 31);
  14. Example of array declaration with a constrained array subtype: 
  15. My_Schedule : Schedule;
--  all arrays of type Schedule have the same bounds
  16. Example of record type with a component that is an array: 
  17. type Var_Line(Length : Natural) is
   record
      Image : String(1 .. Length);
   end record;

  18. Null_Line : Var_Line(0);  --  Null_Line.Image is a null array

Operations of Array Types

Legality Rules

  1. The argument N used in the attribute_designators for the N-th dimension of an array shall be a static expression of some integer type. The value of N shall be positive (nonzero) and no greater than the dimensionality of the array.

    Static Semantics

  2. The following attributes are defined for a prefix A that is of an array type (after any implicit dereference), or denotes a constrained array subtype:
  3. A'First 
    A'First denotes the lower bound of the first index range; its
type is the corresponding index type.
  4. A'First(N) 
    A'First(N) denotes the lower bound of the N-th index range;
its type is the corresponding index type.
  5. A'Last 
    A'Last denotes the upper bound of the first index range; its
type is the corresponding index type.
  6. A'Last(N) 
    A'Last(N) denotes the upper bound of the N-th index range;
its type is the corresponding index type.
  7. A'Range 
    A'Range is equivalent to the range A'First .. A'Last, except
that the prefix A is only evaluated once.
  8. A'Range(N) 
    A'Range(N) is equivalent to the range A'First(N) ..
A'Last(N), except that the prefix A is only evaluated once.
  9. A'Length 
    A'Length denotes the number of values of the first index
range (zero for a null range); its type is universal_integer.
  10. A'Length(N) 
    A'Length(N) denotes the number of values of the N-th index
range (zero for a null range); its type is universal_integer.

    Implementation Advice

  11. An implementation should normally represent multidimensional arrays in row-major order, consistent with the notation used for multidimensional array aggregates, see section Array Aggregates. However, if a pragma Convention(Fortran, ...) applies to a multidimensional array type, then column-major order should be used instead, see section Interfacing with Fortran. NOTES
  12. (45) The attribute_references A'First and A'First(1) denote the same value. A similar relation exists for the attribute_references A'Last, A'Range, and A'Length. The following relation is satisfied (except for a null array) by the above attributes if the index type is an integer type: 
  13. A'Length(N) = A'Last(N) - A'First(N) + 1
  14. (46) An array type is limited if its component type is limited, see section Limited Types.
  15. (47) The predefined operations of an array type include the membership tests, qualification, and explicit conversion. If the array type is not limited, they also include assignment and the predefined equality operators. For a one-dimensional array type, they include the predefined concatenation operators (if nonlimited) and, if the component type is discrete, the predefined relational operators; if the component type is boolean, the predefined logical operators are also included.
  16. (48) A component of an array can be named with an indexed_component. A value of an array type can be specified with an array_aggregate, unless the array type is limited. For a one-dimensional array type, a slice of the array can be named; also, string literals are defined if the component type is a character type.

    Examples

  17. Examples (using arrays declared in the examples of subclause section Index Constraints and Discrete Ranges): 
  18. --  Filter'First       =   0
--  Filter'Last        =  31
--  Filter'Length      =  32
--  Rectangle'Last(1)  =  20
--  Rectangle'Last(2)  =  30

String Types

Static Semantics

  1. A one-dimensional array type whose component type is a character type is called a string type.
  2. There are two predefined string types, String and Wide_String, each indexed by values of the predefined subtype Positive; these are declared in the visible part of package Standard: 
  3. subtype Positive is Integer range 1 .. Integer'Last;

  4. type String is array(Positive range <>) of Character;
type Wide_String is array(Positive range <>) of Wide_Character;
    NOTES
  5. (49) String literals, see section String Literals, and section Literals, are defined for all string types. The concatenation operator & is predefined for string types, as for all nonlimited one-dimensional array types. The ordering operators <, <=, >, and >= are predefined for string types, as for all one-dimensional discrete array types; these ordering operators correspond to lexicographic order, see section Relational Operators and Membership Tests.

    Examples

  6. Examples of string objects: 
  7. Stars      : String(1 .. 120) := (1 .. 120 => '*' );
Question   : constant String  := "How many characters?";
--  Question'First = 1, Question'Last = 20
--  Question'Length = 20 (the number of characters)

  8. Ask_Twice  : String  := Question & Question;
--  constrained to (1..40)

Ninety_Six : constant Roman   := "XCVI";
--  see section Character Types, and section Array Types

Discriminants

  1. A composite type (other than an array type) can have discriminants, which parameterize the type. A known_discriminant_part specifies the discriminants of a composite type. A discriminant of an object is a component of the object, and is either of a discrete type or an access type. An unknown_discriminant_part in the declaration of a partial view of a type specifies that the discriminants of the type are unknown for the given view; all subtypes of such a partial view are indefinite subtypes.

    Syntax 

  2. discriminant_part ::=
   unknown_discriminant_part | known_discriminant_part

  3. unknown_discriminant_part ::= (<>)

  4. known_discriminant_part ::=
   (discriminant_specification {; discriminant_specification})

  5. discriminant_specification ::=
     defining_identifier_list : subtype_mark
       [:= default_expression]
   | defining_identifier_list : access_definition
       [:= default_expression]

  6. default_expression ::= expression

    Name Resolution Rules

  7. The expected type for the default_expression of a discriminant_specification is that of the corresponding discriminant.

    Legality Rules

  8. A known_discriminant_part is only permitted in a declaration for a composite type that is not an array type (this includes generic formal types); a type declared with a known_discriminant_part is called a discriminated type, as is a type that inherits (known) discriminants.
  9. The subtype of a discriminant may be defined by a subtype_mark, in which case the subtype_mark shall denote a discrete or access subtype, or it may be defined by an access_definition (in which case the subtype_mark of the access_definition may denote any kind of subtype). A discriminant that is defined by an access_definition is called an access discriminant and is of an anonymous general access-to-variable type whose designated subtype is denoted by the subtype_mark of the access_definition.
  10. A discriminant_specification for an access discriminant shall appear only in the declaration for a task or protected type, or for a type with the reserved word limited in its (full) definition or in that of one of its ancestors. In addition to the places where Legality Rules normally apply, see section Generic Instantiation, this rule applies also in the private part of an instance of a generic unit.
  11. Default_expressions shall be provided either for all or for none of the discriminants of a known_discriminant_part. No default_expressions are permitted in a known_discriminant_part in a declaration of a tagged type or a generic formal type.
  12. For a type defined by a derived_type_definition, if a known_discriminant_part is provided in its declaration, then:
    1. The parent subtype shall be constrained;
    2. If the parent type is not a tagged type, then each discriminant of the derived type shall be used in the constraint defining the parent subtype;
    3. If a discriminant is used in the constraint defining the parent subtype, the subtype of the discriminant shall be statically compatible (see section Statically Matching Constraints and Subtypes) with the subtype of the corresponding parent discriminant.
  1. The type of the default_expression, if any, for an access discriminant shall be convertible to the anonymous access type of the discriminant (see section Type Conversions).

    Static Semantics

  2. A discriminant_specification declares a discriminant; the subtype_mark denotes its subtype unless it is an access discriminant, in which case the discriminant's subtype is the anonymous access-to-variable subtype defined by the access_definition.
  3. For a type defined by a derived_type_definition, each discriminant of the parent type is either inherited, constrained to equal some new discriminant of the derived type, or constrained to the value of an expression. When inherited or constrained to equal some new discriminant, the parent discriminant and the discriminant of the derived type are said to correspond. Two discriminants also correspond if there is some common discriminant to which they both correspond. A discriminant corresponds to itself as well. If a discriminant of a parent type is constrained to a specific value by a derived_type_definition, then that discriminant is said to be specified by that derived_type_definition.
  4. A constraint that appears within the definition of a discriminated type depends on a discriminant of the type if it names the discriminant as a bound or discriminant value. A component_definition depends on a discriminant if its constraint depends on the discriminant, or on a discriminant that corresponds to it.
  5. A component depends on a discriminant if:
    1. Its component_definition depends on the discriminant; or
    2. It is declared in a variant_part that is governed by the discriminant; or
    3. It is a component inherited as part of a derived_type_definition, and the constraint of the parent_subtype_indication depends on the discriminant; or
    4. It is a subcomponent of a component that depends on the discriminant.
  1. Each value of a discriminated type includes a value for each component of the type that does not depend on a discriminant; this includes the discriminants themselves. The values of discriminants determine which other component values are present in the value of the discriminated type.
  2. A type declared with a known_discriminant_part is said to have known discriminants; its first subtype is unconstrained. A type declared with an unknown_discriminant_part is said to have unknown discriminants. A type declared without a discriminant_part has no discriminants, unless it is a derived type; if derived, such a type has the same sort of discriminants (known, unknown, or none) as its parent (or ancestor) type. A tagged class-wide type also has unknown discriminants. Any subtype of a type with unknown discriminants is an unconstrained and indefinite subtype, see section Types and Subtypes, and section Objects and Named Numbers.

    Dynamic Semantics

  3. An access_definition is elaborated when the value of a corresponding access discriminant is defined, either by evaluation of its default_expression or by elaboration of a discriminant_constraint. The elaboration of an access_definition creates the anonymous access type. When the expression defining the access discriminant is evaluated, it is converted to this anonymous access type, see section Type Conversions. NOTES
  4. (50) If a discriminated type has default_expressions for its discriminants, then unconstrained variables of the type are permitted, and the values of the discriminants can be changed by an assignment to such a variable. If defaults are not provided for the discriminants, then all variables of the type are constrained, either by explicit constraint or by their initial value; the values of the discriminants of such a variable cannot be changed after initialization.
  5. (51) The default_expression for a discriminant of a type is evaluated when an object of an unconstrained subtype of the type is created.
  6. (52) Assignment to a discriminant of an object (after its initialization) is not allowed, since the name of a discriminant is a constant; neither assignment_statements nor assignments inherent in passing as an in out or out parameter are allowed. Note however that the value of a discriminant can be changed by assigning to the enclosing object, presuming it is an unconstrained variable.
  7. (53) A discriminant that is of a named access type is not called an access discriminant; that term is used only for discriminants defined by an access_definition.

    Examples

  8. Examples of discriminated types: 
  9. type Buffer(Size : Buffer_Size := 100) is  --  see section Integer Types
   record
      Pos   : Buffer_Size := 0;
      Value : String(1 .. Size);
   end record;

  10. type Matrix_Rec(Rows, Columns : Integer) is
   record
      Mat : Matrix(1 .. Rows, 1 .. Columns);  --  see section Array Types
   end record;

  11. type Square(Side : Integer) is new
  Matrix_Rec(Rows => Side, Columns => Side);

  12. type Double_Square(Number : Integer) is
   record
      Left  : Square(Number);
      Right : Square(Number);
   end record;

    type Item(Number : Positive) is
   record
      Content : Integer;
      --  no component depends on the discriminant
   end record;

Discriminant Constraints

  1. A discriminant_constraint specifies the values of the discriminants for a given discriminated type.

    Syntax 

  2. discriminant_constraint ::=
   (discriminant_association {, discriminant_association})

  3. discriminant_association ::=
   [discriminant_selector_name
     {| discriminant_selector_name} =>] expression
    1. A discriminant_association is said to be named if it has one or more discriminant_selector_names; it is otherwise said to be positional. In a discriminant_constraint, any positional associations shall precede any named associations.

Name Resolution Rules

  1. Each selector_name of a named discriminant_association shall resolve to denote a discriminant of the subtype being constrained; the discriminants so named are the associated discriminants of the named association. For a positional association, the associated discriminant is the one whose discriminant_specification occurred in the corresponding position in the known_discriminant_part that defined the discriminants of the subtype being constrained.
  2. The expected type for the expression in a discriminant_association is that of the associated discriminant(s).

    Legality Rules

  3. A discriminant_constraint is only allowed in a subtype_indication whose subtype_mark denotes either an unconstrained discriminated subtype, or an unconstrained access subtype whose designated subtype is an unconstrained discriminated subtype.
  4. A named discriminant_association with more than one selector_name is allowed only if the named discriminants are all of the same type. A discriminant_constraint shall provide exactly one value for each discriminant of the subtype being constrained.
  5. The expression associated with an access discriminant shall be of a type convertible to the anonymous access type.

    Dynamic Semantics

  6. A discriminant_constraint is compatible with an unconstrained discriminated subtype if each discriminant value belongs to the subtype of the corresponding discriminant.
  7. A composite value satisfies a discriminant constraint if and only if each discriminant of the composite value has the value imposed by the discriminant constraint.
  8. For the elaboration of a discriminant_constraint, the expressions in the discriminant_associations are evaluated in an arbitrary order and converted to the type of the associated discriminant (which might raise Constraint_Error -- see section Type Conversions.); the expression of a named association is evaluated (and converted) once for each associated discriminant. The result of each evaluation and conversion is the value imposed by the constraint for the associated discriminant. NOTES
  9. (54) The rules of the language ensure that a discriminant of an object always has a value, either from explicit or implicit initialization.

    Examples

  10. Examples (using types declared above in clause section Discriminants): 
  11. Large   : Buffer(200);  --  constrained, always 200 characters
                        --   (explicit discriminant value)
Message : Buffer;       --  unconstrained, initially 100 characters
                        --   (default discriminant value)
Basis   : Square(5);    --  constrained, always 5 by 5
Illegal : Square;       --  illegal, a Square has to be constrained

Operations of Discriminated Types

  1. If a discriminated type has default_expressions for its discriminants, then unconstrained variables of the type are permitted, and the discriminants of such a variable can be changed by assignment to the variable. For a formal parameter of such a type, an attribute is provided to determine whether the corresponding actual parameter is constrained or unconstrained.

    Static Semantics

  2. For a prefix A that is of a discriminated type (after any implicit dereference), the following attribute is defined:
  3. A'Constrained 
    Yields the value True if A denotes a constant, a value, or a
constrained variable, and False otherwise.

    Erroneous Execution

  4. The execution of a construct is erroneous if the construct has a constituent that is a name denoting a subcomponent that depends on discriminants, and the value of any of these discriminants is changed by this execution between evaluating the name and the last use (within this execution) of the subcomponent denoted by the name.

Record Types

  1. A record object is a composite object consisting of named components. The value of a record object is a composite value consisting of the values of the components.

    Syntax 

  2. record_type_definition ::=
   [[abstract] tagged] [limited] record_definition

  3. record_definition ::=
     record
        component_list
     end record
   | null record

  4. component_list ::=
     component_item {component_item}
   | {component_item} variant_part
   |  null;

  5. component_item ::= component_declaration | representation_clause

  6. component_declaration ::=
   defining_identifier_list : component_definition
     [:= default_expression];

    Name Resolution Rules

  7. The expected type for the default_expression, if any, in a component_declaration is the type of the component.

    Legality Rules

  8. A default_expression is not permitted if the component is of a limited type.
  9. Each component_declaration declares a component of the record type. Besides components declared by component_declarations, the components of a record type include any components declared by discriminant_specifications of the record type declaration. The identifiers of all components of a record type shall be distinct.
  10. Within a type_declaration, a name that denotes a component, protected subprogram, or entry of the type is allowed only in the following cases:
    1. A name that denotes any component, protected subprogram, or entry is allowed within a representation item that occurs within the declaration of the composite type.
    2. A name that denotes a noninherited discriminant is allowed within the declaration of the type, but not within the discriminant_part. If the discriminant is used to define the constraint of a component, the bounds of an entry family, or the constraint of the parent subtype in a derived_type_definition then its name shall appear alone as a direct_name (not as part of a larger expression or expanded name). A discriminant shall not be used to define the constraint of a scalar component.
  1. If the name of the current instance of a type, see section The Context of Overload Resolution, is used to define the constraint of a component, then it shall appear as a direct_name that is the prefix of an attribute_reference whose result is of an access type, and the attribute_reference shall appear alone.

    Static Semantics

  2. The component_definition of a component_declaration defines the (nominal) subtype of the component. If the reserved word aliased appears in the component_definition, then the component is aliased, see section Access Types.
  3. If the component_list of a record type is defined by the reserved word null and there are no discriminants, then the record type has no components and all records of the type are null records. A record_definition of null record is equivalent to record null; end record.

    Dynamic Semantics

  4. The elaboration of a record_type_definition creates the record type and its first subtype, and consists of the elaboration of the record_definition. The elaboration of a record_definition consists of the elaboration of its component_list, if any.
  5. The elaboration of a component_list consists of the elaboration of the component_items and variant_part, if any, in the order in which they appear. The elaboration of a component_declaration consists of the elaboration of the component_definition.
  6. Within the definition of a composite type, if a component_definition or discrete_subtype_definition, see section Entries and Accept Statements, includes a name that denotes a discriminant of the type, or that is an attribute_reference whose prefix denotes the current instance of the type, the expression containing the name is called a per-object expression, and the constraint being defined is called a per-object constraint. For the elaboration of a component_definition of a component_declaration, if the constraint of the subtype_indication is not a per-object constraint, then the subtype_indication is elaborated. On the other hand, if the constraint is a per-object constraint, then the elaboration consists of the evaluation of any included expression that is not part of a per-object expression. NOTES
  7. (55) A component_declaration with several identifiers is equivalent to a sequence of single component_declarations, as explained in section Object Declarations.
  8. (56) The default_expression of a record component is only evaluated upon the creation of a default-initialized object of the record type (presuming the object has the component, if it is in a variant_part -- see section Object Declarations).
  9. (57) The subtype defined by a component_definition, see section Array Types, has to be a definite subtype.
  10. (58) If a record type does not have a variant_part, then the same components are present in all values of the type.
  11. (59) A record type is limited if it has the reserved word limited in its definition, or if any of its components are limited, see section Limited Types.
  12. (60) The predefined operations of a record type include membership tests, qualification, and explicit conversion. If the record type is nonlimited, they also include assignment and the predefined equality operators.
  13. (61) A component of a record can be named with a selected_component. A value of a record can be specified with a record_aggregate, unless the record type is limited.

    Examples

  14. Examples of record type declarations: 
  15. type Date is
   record
      Day   : Integer range 1 .. 31;
      Month : Month_Name;
      Year  : Integer range 0 .. 4000;
   end record;

  16. type Complex is
   record
      Re : Real := 0.0;
      Im : Real := 0.0;
   end record;
  17. Examples of record variables: 
  18. Tomorrow, Yesterday : Date;
A, B, C : Complex;

  19. -- both components of A, B, and C are implicitly initialized to zero

Variant Parts and Discrete Choices

  1. A record type with a variant_part specifies alternative lists of components. Each variant defines the components for the value or values of the discriminant covered by its discrete_choice_list.

    Syntax 

  2. variant_part ::=
   case discriminant_direct_name is
      variant
      {variant}
   end case;

  3. variant ::=
   when discrete_choice_list =>
      component_list

  4. discrete_choice_list ::= discrete_choice {| discrete_choice}

  5. discrete_choice ::= expression | discrete_range | others

    Name Resolution Rules

  6. The discriminant_direct_name shall resolve to denote a discriminant (called the discriminant of the variant_part) specified in the known_discriminant_part of the full_type_declaration that contains the variant_part. The expected type for each discrete_choice in a variant is the type of the discriminant of the variant_part.

    Legality Rules

  7. The discriminant of the variant_part shall be of a discrete type.
  8. The expressions and discrete_ranges given as discrete_choices in a variant_part shall be static. The discrete_choice others shall appear alone in a discrete_choice_list, and such a discrete_choice_list, if it appears, shall be the last one in the enclosing construct.
  9. A discrete_choice is defined to cover a value in the following cases:
    1. A discrete_choice that is an expression covers a value if the value equals the value of the expression converted to the expected type.
    2. A discrete_choice that is a discrete_range covers all values (possibly none) that belong to the range.
    3. The discrete_choice others covers all values of its expected type that are not covered by previous discrete_choice_lists of the same construct.
  1. A discrete_choice_list covers a value if one of its discrete_choices covers the value.
  2. The possible values of the discriminant of a variant_part shall be covered as follows:
    1. If the discriminant is of a static constrained scalar subtype, then each non-others discrete_choice shall cover only values in that subtype, and each value of that subtype shall be covered by some discrete_choice (either explicitly or by others);
    2. If the type of the discriminant is a descendant of a generic formal scalar type then the variant_part shall have an others discrete_choice;
    3. Otherwise, each value of the base range of the type of the discriminant shall be covered (either explicitly or by others).
  1. Two distinct discrete_choices of a variant_part shall not cover the same value.

    Static Semantics

  2. If the component_list of a variant is specified by null, the variant has no components.
  3. The discriminant of a variant_part is said to govern the variant_part and its variants. In addition, the discriminant of a derived type governs a variant_part and its variants if it corresponds, see section Discriminants, to the discriminant of the variant_part.

    Dynamic Semantics

  4. A record value contains the values of the components of a particular variant only if the value of the discriminant governing the variant is covered by the discrete_choice_list of the variant. This rule applies in turn to any further variant that is, itself, included in the component_list of the given variant.
  5. The elaboration of a variant_part consists of the elaboration of the component_list of each variant in the order in which they appear.

    Examples

  6. Example of record type with a variant part: 
  7. type Device is (Printer, Disk, Drum);
type State  is (Open, Closed);

  8. type Peripheral(Unit : Device := Disk) is
   record
      Status : State;
      case Unit is
         when Printer =>
            Line_Count : Integer range 1 .. Page_Size;
         when others =>
            Cylinder   : Cylinder_Index;
            Track      : Track_Number;
         end case;
      end record;
  9. Examples of record subtypes: 
  10. subtype Drum_Unit is Peripheral(Drum);
subtype Disk_Unit is Peripheral(Disk);
  11. Examples of constrained record variables: 
  12. Writer   : Peripheral(Unit  => Printer);
Archive  : Disk_Unit;

Tagged Types and Type Extensions

  1. Tagged types and type extensions support object-oriented programming, based on inheritance with extension and run-time polymorphism via dispatching operations.

    Static Semantics

  2. A record type or private type that has the reserved word tagged in its declaration is called a tagged type. When deriving from a tagged type, additional components may be defined. As for any derived type, additional primitive subprograms may be defined, and inherited primitive subprograms may be overridden. The derived type is called an extension of the ancestor type, or simply a type extension. Every type extension is also a tagged type, and is either a record extension or a private extension of some other tagged type. A record extension is defined by a derived_type_definition with a record_extension_part. A private extension, which is a partial view of a record extension, can be declared in the visible part of a package, see section Private Types and Private Extensions, or in a generic formal part, see section Formal Private and Derived Types.
  3. An object of a tagged type has an associated (run-time) tag that identifies the specific tagged type used to create the object originally. The tag of an operand of a class-wide tagged type T'Class controls which subprogram body is to be executed when a primitive subprogram of type T is applied to the operand, see section Dispatching Operations of Tagged Types, using a tag to control which body to execute is called dispatching.
  4. The tag of a specific tagged type identifies the full_type_declaration of the type. If a declaration for a tagged type occurs within a generic_package_declaration, then the corresponding type declarations in distinct instances of the generic package are associated with distinct tags. For a tagged type that is local to a generic package body, the language does not specify whether repeated instantiations of the generic body result in distinct tags.
  5. The following language-defined library package exists: 
  6. package Ada.Tags is
    type Tag is private;

  7.     function Expanded_Name(T : Tag) return String;
    function External_Tag(T : Tag) return String;
    function Internal_Tag(External : String) return Tag;

  8.     Tag_Error : exception;

  9. private
   ... -- not specified by the language
end Ada.Tags;
  10. The function Expanded_Name returns the full expanded name of the first subtype of the specific type identified by the tag, in upper case, starting with a root library unit. The result is implementation defined if the type is declared within an unnamed block_statement.
  11. The function External_Tag returns a string to be used in an external representation for the given tag. The call External_Tag(S'Tag) is equivalent to the attribute_reference S'External_Tag, see section Representation Attributes.
  12. The function Internal_Tag returns the tag that corresponds to the given external tag, or raises Tag_Error if the given string is not the external tag for any specific type of the partition.
  13. For every subtype S of a tagged type T (specific or class-wide), the following attributes are defined:
  14. S'Class 
    S'Class denotes a subtype of the class-wide type (called
T'Class in this International Standard) for the class rooted
at T (or if S already denotes a class-wide subtype, then
S'Class is the same as S).
    1. S'Class is unconstrained. However, if S is constrained, then the values of S'Class are only those that when converted to the type T belong to S.
  1. S'Tag 
    S'Tag denotes the tag of the type T (or if T is class-wide,
the tag of the root type of the corresponding class). The
value of this attribute is of type Tag.
  2. Given a prefix X that is of a class-wide tagged type (after any implicit dereference), the following attribute is defined:
  3. X'Tag 
    X'Tag denotes the tag of X. The value of this attribute is of
type Tag.

    Dynamic Semantics

  4. The tag associated with an object of a tagged type is determined as follows:
    1. The tag of a stand-alone object, a component, or an aggregate of a specific tagged type T identifies T.
    2. The tag of an object created by an allocator for an access type with a specific designated tagged type T, identifies T.
    3. The tag of an object of a class-wide tagged type is that of its initialization expression.
    4. The tag of the result returned by a function whose result type is a specific tagged type T identifies T.
    5. The tag of the result returned by a function with a class-wide result type is that of the return expression.
  1. The tag is preserved by type conversion and by parameter passing. The tag of a value is the tag of the associated object, see section Formal Parameter Modes.

    Implementation Permissions

  2. The implementation of the functions in Ada.Tags may raise Tag_Error if no specific type corresponding to the tag passed as a parameter exists in the partition at the time the function is called. NOTES
  3. (62) A type declared with the reserved word tagged should normally be declared in a package_specification, so that new primitive subprograms can be declared for it.
  4. (63) Once an object has been created, its tag never changes.
  5. (64) Class-wide types are defined to have unknown discriminants (see section Discriminants). This means that objects of a class-wide type have to be explicitly initialized (whether created by an object_declaration or an allocator), and that aggregates have to be explicitly qualified with a specific type when their expected type is class-wide.
  6. (65) If S denotes an untagged private type whose full type is tagged, then S'Class is also allowed before the full type definition, but only in the private part of the package in which the type is declared (see section Private Operations). Similarly, the Class attribute is defined for incomplete types whose full type is tagged, but only within the library unit in which the incomplete type is declared, see section Incomplete Type Declarations.

    Examples

  7. Examples of tagged record types: 
  8. type Point is tagged
  record
    X, Y : Real := 0.0;
  end record;

  9. type Expression is tagged null record;
  -- Components will be added by each extension

Type Extensions

  1. Every type extension is a tagged type, and is either a record extension or a private extension of some other tagged type.

    Syntax 

  2. record_extension_part ::= with record_definition

    Legality Rules

  3. The parent type of a record extension shall not be a class-wide type. If the parent type is nonlimited, then each of the components of the record_extension_part shall be nonlimited. The accessibility level (see section Operations of Access Types) of a record extension shall not be statically deeper than that of its parent type. In addition to the places where Legality Rules normally apply, see section Generic Instantiation, these rules apply also in the private part of an instance of a generic unit.
  4. A type extension shall not be declared in a generic body if the parent type is declared outside that body.

    Dynamic Semantics

  5. The elaboration of a record_extension_part consists of the elaboration of the record_definition. NOTES
  6. (66) The term "type extension" refers to a type as a whole. The term "extension part" refers to the piece of text that defines the additional components (if any) the type extension has relative to its specified ancestor type.
  7. (67) The accessibility rules imply that a tagged type declared in a library package_specification can be extended only at library level or as a generic formal. When the extension is declared immediately within a package_body, primitive subprograms are inherited and are overridable, but new primitive subprograms cannot be added.
  8. (68) A name that denotes a component (including a discriminant) of the parent type is not allowed within the record_extension_part. Similarly, a name that denotes a component defined within the record_extension_part is not allowed within the record_extension_part. It is permissible to use a name that denotes a discriminant of the record extension, providing there is a new known_discriminant_part in the enclosing type declaration. (The full rule is given in section Record Types.)
  9. (69) Each visible component of a record extension has to have a unique name, whether the component is (visibly) inherited from the parent type or declared in the record_extension_part, see section Visibility.

    Examples

  10. Examples of record extensions (of types defined above in section Tagged Types and Type Extensions.): 
  11. type Painted_Point is new Point with
  record
    Paint : Color := White;
  end record;
    -- Components X and Y are inherited

  12. Origin : constant Painted_Point := (X | Y => 0.0, Paint => Black);

  13. type Literal is new Expression with
  record                 -- a leaf in an Expression tree
    Value : Real;
  end record;

  14. type Expr_Ptr is access all Expression'Class;
--  see section Access Types

  15. type Binary_Operation is new Expression with
  record       -- an internal node in an Expression tree
    Left, Right : Expr_Ptr;
  end record;

  16. type Addition is new Binary_Operation with null record;
type Subtraction is new Binary_Operation with null record;
-- No additional components needed for these extensions

  17. Tree : Expr_Ptr :=  -- A tree representation of "5.0 + (13.0-7.0)"
   new Addition'(
      Left  => new Literal'(Value => 5.0),
      Right => new Subtraction'(
         Left  => new Literal'(Value => 13.0),
         Right => new Literal'(Value => 7.0)));

Dispatching Operations of Tagged Types

  1. The primitive subprograms of a tagged type are called dispatching operations. A dispatching operation can be called using a statically determined controlling tag, in which case the body to be executed is determined at compile time. Alternatively, the controlling tag can be dynamically determined, in which case the call dispatches to a body that is determined at run time; such a call is termed a dispatching call. As explained below, the properties of the operands and the context of a particular call on a dispatching operation determine how the controlling tag is determined, and hence whether or not the call is a dispatching call. Run-time polymorphism is achieved when a dispatching operation is called by a dispatching call.

    Static Semantics

  2. A call on a dispatching operation is a call whose name or prefix denotes the declaration of a primitive subprogram of a tagged type, that is, a dispatching operation. A controlling operand in a call on a dispatching operation of a tagged type T is one whose corresponding formal parameter is of type T or is of an anonymous access type with designated type T; the corresponding formal parameter is called a controlling formal parameter. If the controlling formal parameter is an access parameter, the controlling operand is the object designated by the actual parameter, rather than the actual parameter itself. If the call is to a (primitive) function with result type T, then the call has a controlling result -- the context of the call can control the dispatching.
  3. A name or expression of a tagged type is either statically tagged, dynamically tagged, or tag indeterminate, according to whether, when used as a controlling operand, the tag that controls dispatching is determined statically by the operand's (specific) type, dynamically by its tag at run time, or from context. A qualified_expression or parenthesized expression is statically, dynamically, or indeterminately tagged according to its operand. For other kinds of names and expressions, this is determined as follows:
    1. The name or expression is statically tagged if it is of a specific tagged type and, if it is a call with a controlling result, it has at least one statically tagged controlling operand;
    2. The name or expression is dynamically tagged if it is of a class-wide type, or it is a call with a controlling result and at least one dynamically tagged controlling operand;
    3. The name or expression is tag indeterminate if it is a call with a controlling result, all of whose controlling operands (if any) are tag indeterminate.
  1. A type_conversion is statically or dynamically tagged according to whether the type determined by the subtype_mark is specific or class-wide, respectively. For a controlling operand that is designated by an actual parameter, the controlling operand is statically or dynamically tagged according to whether the designated type of the actual parameter is specific or class-wide, respectively.

    Legality Rules

  2. A call on a dispatching operation shall not have both dynamically tagged and statically tagged controlling operands.
  3. If the expected type for an expression or name is some specific tagged type, then the expression or name shall not be dynamically tagged unless it is a controlling operand in a call on a dispatching operation. Similarly, if the expected type for an expression is an anonymous access-to-specific tagged type, then the expression shall not be of an access-to-class-wide type unless it designates a controlling operand in a call on a dispatching operation.
  4. In the declaration of a dispatching operation of a tagged type, everywhere a subtype of the tagged type appears as a subtype of the profile, see section Subprogram Declarations, it shall statically match the first subtype of the tagged type. If the dispatching operation overrides an inherited subprogram, it shall be subtype conformant with the inherited subprogram. A dispatching operation shall not be of convention Intrinsic. If a dispatching operation overrides the predefined equals operator, then it shall be of convention Ada (either explicitly or by default -- see section Conformance Rules.).
  5. The default_expression for a controlling formal parameter of a dispatching operation shall be tag indeterminate. A controlling formal parameter that is an access parameter shall not have a default_expression.
  6. A given subprogram shall not be a dispatching operation of two or more distinct tagged types.
  7. The explicit declaration of a primitive subprogram of a tagged type shall occur before the type is frozen, see section Freezing Rules. For example, new dispatching operations cannot be added after objects or values of the type exist, nor after deriving a record extension from it, nor after a body.

    Dynamic Semantics

  8. For the execution of a call on a dispatching operation of a type T, the controlling tag value determines which subprogram body is executed. The controlling tag value is defined as follows:
    1. If one or more controlling operands are statically tagged, then the controlling tag value is statically determined to be the tag of T.
    2. If one or more controlling operands are dynamically tagged, then the controlling tag value is not statically determined, but is rather determined by the tags of the controlling operands. If there is more than one dynamically tagged controlling operand, a check is made that they all have the same tag. If this check fails, Constraint_Error is raised unless the call is a function_call whose name denotes the declaration of an equality operator (predefined or user defined) that returns Boolean, in which case the result of the call is defined to indicate inequality, and no subprogram_body is executed. This check is performed prior to evaluating any tag-indeterminate controlling operands.
    3. If all of the controlling operands are tag-indeterminate, then:
      1. If the call has a controlling result and is itself a (possibly parenthesized or qualified) controlling operand of an enclosing call on a dispatching operation of type T, then its controlling tag value is determined by the controlling tag value of this enclosing call;
      2. Otherwise, the controlling tag value is statically determined to be the tag of type T.
  1. For the execution of a call on a dispatching operation, the body executed is the one for the corresponding primitive subprogram of the specific type identified by the controlling tag value. The body for an explicitly declared dispatching operation is the corresponding explicit body for the subprogram. The body for an implicitly declared dispatching operation that is overridden is the body for the overriding subprogram, even if the overriding occurs in a private part. The body for an inherited dispatching operation that is not overridden is the body of the corresponding subprogram of the parent or ancestor type. NOTES
  2. (70) The body to be executed for a call on a dispatching operation is determined by the tag; it does not matter whether that tag is determined statically or dynamically, and it does not matter whether the subprogram's declaration is visible at the place of the call.
  3. (71) This subclause covers calls on primitive subprograms of a tagged type. Rules for tagged type membership tests are described in 4.5.2. Controlling tag determination for an assignment_statement is described in section Assignment Statements.
  4. (72) A dispatching call can dispatch to a body whose declaration is not visible at the place of the call.
  5. (73) A call through an access-to-subprogram value is never a dispatching call, even if the access value designates a dispatching operation. Similarly a call whose prefix denotes a subprogram_renaming_declaration cannot be a dispatching call unless the renaming itself is the declaration of a primitive subprogram.

Abstract Types and Subprograms

  1. An abstract type is a tagged type intended for use as a parent type for type extensions, but which is not allowed to have objects of its own. An abstract subprogram is a subprogram that has no body, but is intended to be overridden at some point when inherited. Because objects of an abstract type cannot be created, a dispatching call to an abstract subprogram always dispatches to some overriding body.

    Legality Rules

  2. An abstract type is a specific type that has the reserved word abstract in its declaration. Only a tagged type is allowed to be declared abstract.
  3. A subprogram declared by an abstract_subprogram_declaration, see section Subprogram Declarations, is an abstract subprogram. If it is a primitive subprogram of a tagged type, then the tagged type shall be abstract.
  4. For a derived type, if the parent or ancestor type has an abstract primitive subprogram, or a primitive function with a controlling result, then:
    1. If the derived type is abstract or untagged, the inherited subprogram is abstract.
    2. Otherwise, the subprogram shall be overridden with a nonabstract subprogram; for a type declared in the visible part of a package, the overriding may be either in the visible or the private part. However, if the type is a generic formal type, the subprogram need not be overridden for the formal type itself; a nonabstract version will necessarily be provided by the actual type.
  1. A call on an abstract subprogram shall be a dispatching call; nondispatching calls to an abstract subprogram are not allowed.
  2. The type of an aggregate, or of an object created by an object_declaration or an allocator, or a generic formal object of mode in, shall not be abstract. The type of the target of an assignment operation, see section Assignment Statements, shall not be abstract. The type of a component shall not be abstract. If the result type of a function is abstract, then the function shall be abstract.
  3. If a partial view is not abstract, the corresponding full view shall not be abstract. If a generic formal type is abstract, then for each primitive subprogram of the formal that is not abstract, the corresponding primitive subprogram of the actual shall not be abstract.
  4. For an abstract type declared in a visible part, an abstract primitive subprogram shall not be declared in the private part, unless it is overriding an abstract subprogram implicitly declared in the visible part. For a tagged type declared in a visible part, a primitive function with a controlling result shall not be declared in the private part, unless it is overriding a function implicitly declared in the visible part.
  5. A generic actual subprogram shall not be an abstract subprogram. The prefix of an attribute_reference for the Access, Unchecked_Access, or Address attributes shall not denote an abstract subprogram. NOTES
  6. (74) Abstractness is not inherited; to declare an abstract type, the reserved word abstract has to be used in the declaration of the type extension.
  7. (75) A class-wide type is never abstract. Even if a class is rooted at an abstract type, the class-wide type for the class is not abstract, and an object of the class-wide type can be created; the tag of such an object will identify some nonabstract type in the class.

    Examples

  8. Example of an abstract type representing a set of natural numbers: 
  9. package Sets is
    subtype Element_Type is Natural;
    type Set is abstract tagged null record;
    function Empty return Set is abstract;
    function Union(Left, Right : Set) return Set is abstract;
    function Intersection(Left, Right : Set) return Set is abstract;
    function Unit_Set(Element : Element_Type) return Set is abstract;
    procedure Take
      (Element : out Element_Type;
       From : in out Set) is abstract;
end Sets;
    NOTES
  10. (76) Notes on the example: Given the above abstract type, one could then derive various (nonabstract) extensions of the type, representing alternative implementations of a set. One might use a bit vector, but impose an upper bound on the largest element representable, while another might use a hash table, trading off space for flexibility.

Access Types

  1. A value of an access type (an access value) provides indirect access to the object or subprogram it designates. Depending on its type, an access value can designate either subprograms, objects created by allocators (see section Allocators) or more generally aliased objects of an appropriate type.

    Syntax 

  2. access_type_definition ::=
     access_to_object_definition
   | access_to_subprogram_definition

  3. access_to_object_definition ::=
    access [general_access_modifier] subtype_indication

  4. general_access_modifier ::= all | constant

  5. access_to_subprogram_definition ::=
     access [protected] procedure parameter_profile
   | access [protected] function  parameter_and_result_profile

  6. access_definition ::= access subtype_mark

    Static Semantics

  7. There are two kinds of access types, access-to-object types, whose values designate objects, and access-to-subprogram types, whose values designate subprograms. Associated with an access-to-object type is a storage pool; several access types may share the same storage pool. A storage pool is an area of storage used to hold dynamically allocated objects (called pool elements) created by allocators; storage pools are described further in section Storage Management.
  8. Access-to-object types are further subdivided into pool-specific access types, whose values can designate only the elements of their associated storage pool, and general access types, whose values can designate the elements of any storage pool, as well as aliased objects created by declarations rather than allocators, and aliased subcomponents of other objects.
  9. A view of an object is defined to be aliased if it is defined by an object_declaration or component_definition with the reserved word aliased, or by a renaming of an aliased view. In addition, the dereference of an access-to-object value denotes an aliased view, as does a view conversion, see section Type Conversions of an aliased view. Finally, the current instance of a limited type, and a formal parameter or generic formal object of a tagged type are defined to be aliased. Aliased views are the ones that can be designated by an access value. If the view defined by an object_declaration is aliased, and the type of the object has discriminants, then the object is constrained; if its nominal subtype is unconstrained, then the object is constrained by its initial value. Similarly, if the object created by an allocator has discriminants, the object is constrained, either by the designated subtype, or by its initial value.
  10. An access_to_object_definition defines an access-to-object type and its first subtype; the subtype_indication defines the designated subtype of the access type. If a general_access_modifier appears, then the access type is a general access type. If the modifier is the reserved word constant, then the type is an access-to-constant type; a designated object cannot be updated through a value of such a type. If the modifier is the reserved word all, then the type is an access-to-variable type; a designated object can be both read and updated through a value of such a type. If no general_access_modifier appears in the access_to_object_definition, the access type is a pool-specific access-to-variable type.
  11. An access_to_subprogram_definition defines an access-to-subprogram type and its first subtype; the parameter_profile or parameter_and_result_profile defines the designated profile of the access type. There is a calling convention associated with the designated profile; only subprograms with this calling convention can be designated by values of the access type. By default, the calling convention is "protected" if the reserved word protected appears, and "Ada" otherwise. See Annex section Interface to Other Languages (normative) for how to override this default.
  12. An access_definition defines an anonymous general access-to-variable type; the subtype_mark denotes its designated subtype. An access_definition is used in the specification of an access discriminant, see section Discriminants, or an access parameter, see section Subprogram Declarations.
  13. For each (named) access type, there is a literal null which has a null access value designating no entity at all. The null value of a named access type is the default initial value of the type. Other values of an access type are obtained by evaluating an attribute_reference for the Access or Unchecked_Access attribute of an aliased view of an object or non-intrinsic subprogram, or, in the case of a named access-to-object type, an allocator, which returns an access value designating a newly created object, see section Operations of Access Types.
  14. All subtypes of an access-to-subprogram type are constrained. The first subtype of a type defined by an access_type_definition or an access_to_object_definition is unconstrained if the designated subtype is an unconstrained array or discriminated type; otherwise it is constrained.

    Dynamic Semantics

  15. A composite_constraint is compatible with an unconstrained access subtype if it is compatible with the designated subtype. An access value satisfies a composite_constraint of an access subtype if it equals the null value of its type or if it designates an object whose value satisfies the constraint.
  16. The elaboration of an access_type_definition creates the access type and its first subtype. For an access-to-object type, this elaboration includes the elaboration of the subtype_indication, which creates the designated subtype.
  17. The elaboration of an access_definition creates an anonymous general access-to-variable type (this happens as part of the initialization of an access parameter or access discriminant). NOTES
  18. (77) Access values are called "pointers" or "references" in some other languages.
  19. (78) Each access-to-object type has an associated storage pool; several access types can share the same pool. An object can be created in the storage pool of an access type by an allocator, see section Allocators, for the access type. A storage pool (roughly) corresponds to what some other languages call a "heap.". See section Storage Management, for a discussion of pools.
  20. (79) Only index_constraints and discriminant_constraints can be applied to access types, see section Index Constraints and Discrete Ranges, and section Discriminant Constraints.

    Examples

  21. Examples of access-to-object types: 
  22. type Peripheral_Ref is access Peripheral;  --  see section Variant Parts and Discrete Choices
type Binop_Ptr is access all Binary_Operation'Class;
-- general access-to-class-wide, see section Type Extensions
  23. Example of an access subtype: 
  24. subtype Drum_Ref is Peripheral_Ref(Drum);  --  see section Variant Parts and Discrete Choices
  25. Example of an access-to-subprogram type: 
  26. type Message_Procedure is access
  procedure (M : in String := "Error!");
procedure Default_Message_Procedure(M : in String);
Give_Message : Message_Procedure := Default_Message_Procedure'Access;
...
procedure Other_Procedure(M : in String);
...
Give_Message := Other_Procedure'Access;
...
Give_Message("File not found.");
-- call with parameter (.all is optional)
Give_Message.all;
-- call with no parameters

Incomplete Type Declarations

  1. There are no particular limitations on the designated type of an access type. In particular, the type of a component of the designated type can be another access type, or even the same access type. This permits mutually dependent and recursive access types. An incomplete_type_declaration can be used to introduce a type to be used as a designated type, while deferring its full definition to a subsequent full_type_declaration.

    Syntax 

  2. incomplete_type_declaration ::=
   type defining_identifier [discriminant_part];

    Legality Rules

  3. An incomplete_type_declaration requires a completion, which shall be a full_type_declaration. If the incomplete_type_declaration occurs immediately within either the visible part of a package_specification or a declarative_part, then the full_type_declaration shall occur later and immediately within this visible part or declarative_part. If the incomplete_type_declaration occurs immediately within the private part of a given package_specification, then the full_type_declaration shall occur later and immediately within either the private part itself, or the declarative_part of the corresponding package_body.
  4. If an incomplete_type_declaration has a known_discriminant_part, then a full_type_declaration that completes it shall have a fully conforming (explicit) known_discriminant_part, see section Conformance Rules. If an incomplete_type_declaration has no discriminant_part (or an unknown_discriminant_part), then a corresponding full_type_declaration is nevertheless allowed to have discriminants, either explicitly, or inherited via derivation.
  5. The only allowed uses of a name that denotes an incomplete_type_declaration are as follows:
    1. as the subtype_mark in the subtype_indication of an access_to_object_definition; the only form of constraint allowed in this subtype_indication is a discriminant_constraint;
    2. as the subtype_mark defining the subtype of a parameter or result of an access_to_subprogram_definition;
    3. as the subtype_mark in an access_definition;
    4. as the prefix of an attribute_reference whose attribute_designator is Class; such an attribute_reference is similarly restricted to the uses allowed here; when used in this way, the corresponding full_type_declaration shall declare a tagged type, and the attribute_reference shall occur in the same library unit as the incomplete_type_declaration.
  1. A dereference (whether implicit or explicit -- see section Names.) shall not be of an incomplete type.

    Static Semantics

  2. An incomplete_type_declaration declares an incomplete type and its first subtype; the first subtype is unconstrained if a known_discriminant_part appears.

    Dynamic Semantics

  3. The elaboration of an incomplete_type_declaration has no effect. NOTES
  4. (80) Within a declarative_part, an incomplete_type_declaration and a corresponding full_type_declaration cannot be separated by an intervening body. This is because a type has to be completely defined before it is frozen, and a body freezes all types declared prior to it in the same declarative_part, see section Freezing Rules.

    Examples

  5. Example of a recursive type: 
  6. type Cell;  --  incomplete type declaration
type Link is access Cell;

  7. type Cell is
   record
      Value  : Integer;
      Succ   : Link;
      Pred   : Link;
   end record;

  8. Head   : Link  := new Cell'(0, null, null);
Next   : Link  := Head.Succ;
  9. Examples of mutually dependent access types: 
  10. type Person(<>);    -- incomplete type declaration
type Car;           -- incomplete type declaration

  11. type Person_Name is access Person;
type Car_Name    is access all Car;

  12. type Car is
   record
      Number  : Integer;
      Owner   : Person_Name;
   end record;

  13. type Person(Sex : Gender) is
   record
      Name     : String(1 .. 20);
      Birth    : Date;
      Age      : Integer range 0 .. 130;
      Vehicle  : Car_Name;
      case Sex is
         when M => Wife    : Person_Name(Sex => F);
         when F => Husband : Person_Name(Sex => M);
      end case;
   end record;

  14. My_Car, Your_Car, Next_Car : Car_Name := new Car;
--  see section Allocators
George : Person_Name := new Person(M);
   ...
George.Vehicle := Your_Car;

Operations of Access Types

  1. The attribute Access is used to create access values designating aliased objects and non-intrinsic subprograms. The "accessibility" rules prevent dangling references (in the absence of uses of certain unchecked features -- see section Representation Issues.).

    Name Resolution Rules

  2. For an attribute_reference with attribute_designator Access (or Unchecked_Access -- see section Unchecked Access Value Creation.), the expected type shall be a single access type; the prefix of such an attribute_reference is never interpreted as an implicit_dereference. If the expected type is an access-to-subprogram type, then the expected profile of the prefix is the designated profile of the access type.

    Static Semantics

  3. The accessibility rules, which prevent dangling references, are written in terms of accessibility levels, which reflect the run-time nesting of masters. As explained in section Completion and Finalization, a master is the execution of a task_body, a block_statement, a subprogram_body, an entry_body, or an accept_statement. An accessibility level is deeper than another if it is more deeply nested at run time. For example, an object declared local to a called subprogram has a deeper accessibility level than an object declared local to the calling subprogram. The accessibility rules for access types require that the accessibility level of an object designated by an access value be no deeper than that of the access type. This ensures that the object will live at least as long as the access type, which in turn ensures that the access value cannot later designate an object that no longer exists. The Unchecked_Access attribute may be used to circumvent the accessibility rules.
  4. A given accessibility level is said to be statically deeper than another if the given level is known at compile time (as defined below) to be deeper than the other for all possible executions. In most cases, accessibility is enforced at compile time by Legality Rules. Run-time accessibility checks are also used, since the Legality Rules do not cover certain cases involving access parameters and generic packages.
  5. Each master, and each entity and view created by it, has an accessibility level:
    1. The accessibility level of a given master is deeper than that of each dynamically enclosing master, and deeper than that of each master upon which the task executing the given master directly depends, see section Task Dependence - Termination of Tasks.
    2. An entity or view created by a declaration has the same accessibility level as the innermost enclosing master, except in the cases of renaming and derived access types described below. A parameter of a master has the same accessibility level as the master.
    3. The accessibility level of a view of an object or subprogram defined by a renaming_declaration is the same as that of the renamed view.
    4. The accessibility level of a view conversion is the same as that of the operand.
    5. For a function whose result type is a return-by-reference type, the accessibility level of the result object is the same as that of the master that elaborated the function body. For any other function, the accessibility level of the result object is that of the execution of the called function.
    6. The accessibility level of a derived access type is the same as that of its ultimate ancestor.
    7. The accessibility level of the anonymous access type of an access discriminant is the same as that of the containing object or associated constrained subtype.
    8. The accessibility level of the anonymous access type of an access parameter is the same as that of the view designated by the actual. If the actual is an allocator, this is the accessibility level of the execution of the called subprogram.
    9. The accessibility level of an object created by an allocator is the same as that of the access type.
    10. The accessibility level of a view of an object or subprogram denoted by a dereference of an access value is the same as that of the access type.
    11. The accessibility level of a component, protected subprogram, or entry of (a view of) a composite object is the same as that of (the view of) the composite object.
  1. One accessibility level is defined to be statically deeper than another in the following cases:
    1. For a master that is statically nested within another master, the accessibility level of the inner master is statically deeper than that of the outer master.
    2. The statically deeper relationship does not apply to the accessibility level of the anonymous type of an access parameter; that is, such an accessibility level is not considered to be statically deeper, nor statically shallower, than any other.
    3. For determining whether one level is statically deeper than another when within a generic package body, the generic package is presumed to be instantiated at the same level as where it was declared; run-time checks are needed in the case of more deeply nested instantiations.
    4. For determining whether one level is statically deeper than another when within the declarative region of a type_declaration, the current instance of the type is presumed to be an object created at a deeper level than that of the type.
  1. The accessibility level of all library units is called the library level; a library-level declaration or entity is one whose accessibility level is the library level.
  2. The following attribute is defined for a prefix X that denotes an aliased view of an object:
  3. X'Access 
    X'Access yields an access value that designates the object
denoted by X. The type of X'Access is an access-to-object
type, as determined by the expected type. The expected type
shall be a general access type. X shall denote an aliased
view of an object, including possibly the current instance
(see section The Context of Overload Resolution) of a limited type within its definition, or a
formal parameter or generic formal object of a tagged type.
The view denoted by the prefix X shall satisfy the following
additional requirements, presuming the expected type for
X'Access is the general access type A:
      1. If A is an access-to-variable type, then the view shall be a variable; on the other hand, if A is an access-to-constant type, the view may be either a constant or a variable.
      2. The view shall not be a subcomponent that depends on discriminants of a variable whose nominal subtype is unconstrained, unless this subtype is indefinite, or the variable is aliased.
      3. If the designated type of A is tagged, then the type of the view shall be covered by the designated type; if A's designated type is not tagged, then the type of the view shall be the same, and either A's designated subtype shall statically match the nominal subtype of the view, or the designated subtype shall be discriminated and unconstrained;
      4. The accessibility level of the view shall not be statically deeper than that of the access type A. In addition to the places where Legality Rules normally apply, see section Generic Instantiation, this rule applies also in the private part of an instance of a generic unit.
    1. A check is made that the accessibility level of X is not deeper than that of the access type A. If this check fails, Program_Error is raised.
    2. If the nominal subtype of X does not statically match the designated subtype of A, a view conversion of X to the designated subtype is evaluated (which might raise Constraint_Error -- see section Type Conversions.) and the value of X'Access designates that view.
  1. The following attribute is defined for a prefix P that denotes a subprogram:
  2. P'Access 
    P'Access yields an access value that designates the
subprogram denoted by P. The type of P'Access is an
access-to-subprogram type (S), as determined by the expected
type. The accessibility level of P shall not be statically
deeper than that of S. In addition to the places where
Legality Rules normally apply, see section Generic Instantiation, this rule applies
also in the private part of an instance of a generic unit.
The profile of P shall be subtype-conformant with the
designated profile of S, and shall not be Intrinsic. If
the subprogram denoted by P is declared within a generic
body, S shall be declared within the generic body.
    NOTES
  3. (81) The Unchecked_Access attribute yields the same result as the Access attribute for objects, but has fewer restrictions, see section Unchecked Access Value Creation. There are other predefined operations that yield access values: an allocator can be used to create an object, and return an access value that designates it, see section Allocators, evaluating the literal null yields a null access value that designates no entity at all, see section Literals.
  4. (82) The predefined operations of an access type also include the assignment operation, qualification, and membership tests. Explicit conversion is allowed between general access types with matching designated subtypes; explicit conversion is allowed between access-to-subprogram types with subtype conformant profiles, see section Type Conversions. Named access types have predefined equality operators; anonymous access types do not, see section Relational Operators and Membership Tests.
  5. (83) The object or subprogram designated by an access value can be named with a dereference, either an explicit_dereference or an implicit_dereference (see section Names).
  6. (84) A call through the dereference of an access-to-subprogram value is never a dispatching call.
  7. (85) The accessibility rules imply that it is not possible to use the Access attribute to implement "downward closures" -- that is, to pass a more-nested subprogram as a parameter to a less-nested subprogram, as might be desired for example for an iterator abstraction. Instead, downward closures can be implemented using generic formal subprograms (see section Formal Subprograms). Note that Unchecked_Access is not allowed for subprograms.
  8. (86) Note that using an access-to-class-wide tagged type with a dispatching operation is a potentially more structured alternative to using an access-to-subprogram type.
  9. (87) An implementation may consider two access-to-subprogram values to be unequal, even though they designate the same subprogram. This might be because one points directly to the subprogram, while the other points to a special prologue that performs an Elaboration_Check and then jumps to the subprogram (see section Relational Operators and Membership Tests).

    Examples

  10. Example of use of the Access attribute: 
  11. Martha : Person_Name := new Person(F);  --  see section Incomplete Type Declarations
Cars   : array (1..2) of aliased Car;
   ...
Martha.Vehicle := Cars(1)'Access;
George.Vehicle := Cars(2)'Access;

Declarative Parts

  1. A declarative_part contains declarative_items (possibly none).

    Syntax 

  2. declarative_part ::= {declarative_item}

  3. declarative_item ::= basic_declarative_item | body

  4. basic_declarative_item ::=
   basic_declaration | representation_clause | use_clause

  5. body ::= proper_body | body_stub

  6. proper_body ::=
   subprogram_body | package_body | task_body | protected_body

    Dynamic Semantics

  7. The elaboration of a declarative_part consists of the elaboration of the declarative_items, if any, in the order in which they are given in the declarative_part.
  8. An elaborable construct is in the elaborated state after the normal completion of its elaboration. Prior to that, it is not yet elaborated.
  9. For a construct that attempts to use a body, a check (Elaboration_Check) is performed, as follows:
    1. For a call to a (non-protected) subprogram that has an explicit body, a check is made that the subprogram_body is already elaborated. This check and the evaluations of any actual parameters of the call are done in an arbitrary order.
    2. For a call to a protected operation of a protected type (that has a body -- no check is performed if a pragma Import applies to the protected type), a check is made that the protected_body is already elaborated. This check and the evaluations of any actual parameters of the call are done in an arbitrary order.
    3. For the activation of a task, a check is made by the activator that the task_body is already elaborated. If two or more tasks are being activated together, see section Task Execution - Task Activation, as the result of the elaboration of a declarative_part or the initialization for the object created by an allocator, this check is done for all of them before activating any of them.
    4. For the instantiation of a generic unit that has a body, a check is made that this body is already elaborated. This check and the evaluation of any explicit_generic_actual_parameters of the instantiation are done in an arbitrary order.
  1. The exception Program_Error is raised if any of these checks fails.

Completions of Declarations

  1. Declarations sometimes come in two parts. A declaration that requires a second part is said to require completion. The second part is called the completion of the declaration (and of the entity declared), and is either another declaration, a body, or a pragma.

    Name Resolution Rules

  2. A construct that can be a completion is interpreted as the completion of a prior declaration only if:
    1. The declaration and the completion occur immediately within the same declarative region;
    2. The defining name or defining_program_unit_name in the completion is the same as in the declaration, or in the case of a pragma, the pragma applies to the declaration;
    3. If the declaration is overloadable, then the completion either has a type-conformant profile, or is a pragma.

Legality Rules

  1. An implicit declaration shall not have a completion. For any explicit declaration that is specified to require completion, there shall be a corresponding explicit completion.
  2. At most one completion is allowed for a given declaration. Additional requirements on completions appear where each kind of completion is defined.
  3. A type is completely defined at a place that is after its full type definition (if it has one) and after all of its subcomponent types are completely defined. A type shall be completely defined before it is frozen, see section Freezing Rules, and section Private Types and Private Extensions. NOTES
  4. (88) Completions are in principle allowed for any kind of explicit declaration. However, for some kinds of declaration, the only allowed completion is a pragma Import, and implementations are not required to support pragma Import for every kind of entity.
  5. (89) There are rules that prevent premature uses of declarations that have a corresponding completion. The Elaboration_Checks (see section Declarative Parts) prevent such uses at run time for subprograms, protected operations, tasks, and generic units. The "Freezing Rules" (see section Freezing Rules) prevent, at compile time, premature uses of other entities such as private types and deferred constants.

Names and Expressions

  1. The rules applicable to the different forms of name and expression, and to their evaluation, are given in this section.

Names

  1. Names can denote declared entities, whether declared explicitly or implicitly, see section Declarations. Names can also denote objects or subprograms designated by access values; the results of type_conversions or function_calls; subcomponents and slices of objects and values; protected subprograms, single entries, entry families, and entries in families of entries. Finally, names can denote attributes of any of the foregoing.

    Syntax 

  2. name ::=
     direct_name          | explicit_dereference
   | indexed_component    | slice
   | selected_component   | attribute_reference
   | type_conversion      | function_call
   | character_literal

  3. direct_name ::= identifier | operator_symbol

  4. prefix ::= name | implicit_dereference

  5. explicit_dereference ::= name.all

  6. implicit_dereference ::= name
  7. Certain forms of name (indexed_components, selected_components, slices, and attributes) include a prefix that is either itself a name that denotes some related entity, or an implicit_dereference of an access value that designates some related entity.

    Name Resolution Rules

  8. The name in a dereference (either an implicit_dereference or an explicit_dereference) is expected to be of any access type.

    Static Semantics

  9. If the type of the name in a dereference is some access-to-object type T, then the dereference denotes a view of an object, the nominal subtype of the view being the designated subtype of T.
  10. If the type of the name in a dereference is some access-to-subprogram type S, then the dereference denotes a view of a subprogram, the profile of the view being the designated profile of S.

    Dynamic Semantics

  11. The evaluation of a name determines the entity denoted by the name. This evaluation has no other effect for a name that is a direct_name or a character_literal.
  12. The evaluation of a name that has a prefix includes the evaluation of the prefix. The evaluation of a prefix consists of the evaluation of the name or the implicit_dereference. The prefix denotes the entity denoted by the name or the implicit_dereference.
  13. The evaluation of a dereference consists of the evaluation of the name and the determination of the object or subprogram that is designated by the value of the name. A check is made that the value of the name is not the null access value. Constraint_Error is raised if this check fails. The dereference denotes the object or subprogram designated by the value of the name.

    Examples

  14. Examples of direct names: 
  15. Pi      -- the direct name of a number            (see section Number Declarations)
Limit   -- the direct name of a constant          (see section Object Declarations)
Count   -- the direct name of a scalar variable   (see section Object Declarations)
Board   -- the direct name of an array variable   (see section Index Constraints and Discrete Ranges)
Matrix  -- the direct name of a type              (see section Array Types)
Random  -- the direct name of a function          (see section Subprogram Declarations)
Error   -- the direct name of an exception        (see section Exception Declarations)
  16. Examples of dereferences: 
  17. Next_Car.all    --  explicit dereference denoting the object
                --  designated by the access variable Next_Car,
                --  see section Incomplete Type Declarations
Next_Car.Owner  --  selected component with implicit dereference;
                --  same as Next_Car.all.Owner

Indexed Components

  1. An indexed_component denotes either a component of an array or an entry in a family of entries.

    Syntax 

  2. indexed_component ::= prefix(expression {, expression})

    Name Resolution Rules

  3. The prefix of an indexed_component with a given number of expressions shall resolve to denote an array (after any implicit dereference) with the corresponding number of index positions, or shall resolve to denote an entry family of a task or protected object (in which case there shall be only one expression).
  4. The expected type for each expression is the corresponding index type.

    Static Semantics

  5. When the prefix denotes an array, the indexed_component denotes the component of the array with the specified index value(s). The nominal subtype of the indexed_component is the component subtype of the array type.
  6. When the prefix denotes an entry family, the indexed_component denotes the individual entry of the entry family with the specified index value.

    Dynamic Semantics

  7. For the evaluation of an indexed_component, the prefix and the expressions are evaluated in an arbitrary order. The value of each expression is converted to the corresponding index type. A check is made that each index value belongs to the corresponding index range of the array or entry family denoted by the prefix. Constraint_Error is raised if this check fails.

    Examples

  8. Examples of indexed components: 
  9. My_Schedule(Sat)
--  a component of a one-dimensional array   (see see section Index Constraints and Discrete Ranges)

Page(10)
--  a component of a one-dimensional array   (see see section Array Types)

Board(M, J + 1)
--  a component of a two-dimensional array   (see see section Index Constraints and Discrete Ranges)

Page(10)(20)
--  a component of a component               (see see section Array Types)

Request(Medium)
--  an entry in a family of entries          (see see section Task Units and Task Objects)

Next_Frame(L)(M, N)
--  a component of a function call           (see see section Subprogram Declarations)
    NOTES
  10. (1) Notes on the examples: Distinct notations are used for components of multidimensional arrays (such as Board) and arrays of arrays (such as Page). The components of an array of arrays are arrays and can therefore be indexed. Thus Page(10)(20) denotes the 20th component of Page(10). In the last example Next_Frame(L) is a function call returning an access value that designates a two-dimensional array.

Slices

  1. A slice denotes a one-dimensional array formed by a sequence of consecutive components of a one-dimensional array. A slice of a variable is a variable; a slice of a constant is a constant; a slice of a value is a value.

    Syntax 

  2. slice ::= prefix(discrete_range)

    Name Resolution Rules

  3. The prefix of a slice shall resolve to denote a one-dimensional array (after any implicit dereference).
  4. The expected type for the discrete_range of a slice is the index type of the array type.

    Static Semantics

  5. A slice denotes a one-dimensional array formed by the sequence of consecutive components of the array denoted by the prefix, corresponding to the range of values of the index given by the discrete_range.
  6. The type of the slice is that of the prefix. Its bounds are those defined by the discrete_range.

    Dynamic Semantics

  7. For the evaluation of a slice, the prefix and the discrete_range are evaluated in an arbitrary order. If the slice is not a null slice (a slice where the discrete_range is a null range), then a check is made that the bounds of the discrete_range belong to the index range of the array denoted by the prefix. Constraint_Error is raised if this check fails. NOTES
  8. (2) A slice is not permitted as the prefix of an Access attribute_reference, even if the components or the array as a whole are aliased (see section Operations of Access Types).
  9. (3) For a one-dimensional array A, the slice A(N .. N) denotes an array that has only one component; its type is the type of A. On the other hand, A(N) denotes a component of the array A and has the corresponding component type.

    Examples

  10. Examples of slices: 
  11. Stars(1 .. 15)
--  a slice of 15 characters         (see section String Types)

Page(10 .. 10 + Size)
--  a slice of 1 + Size components   (see section Array Types)

Page(L)(A .. B)
--  a slice of the array Page(L)     (see section Array Types)

Stars(1 .. 0)
--  a null slice                     (see section String Types)

My_Schedule(Weekday)
--  bounds given by subtype          (see section Index Constraints and Discrete Ranges, and section Enumeration Types)

Stars(5 .. 15)(K)
--  same as Stars(K)                 (see section String Types)
--  provided that K is in 5 .. 15

Selected Components

  1. Selected_components are used to denote components (including discriminants), entries, entry families, and protected subprograms; they are also used as expanded names as described below.

    Syntax 

  2. selected_component ::= prefix . selector_name

  3. selector_name ::= identifier | character_literal | operator_symbol

    Name Resolution Rules

  4. A selected_component is called an expanded name if, according to the visibility rules, at least one possible interpretation of its prefix denotes a package or an enclosing named construct (directly, not through a subprogram_renaming_declaration or generic_renaming_declaration).
  5. A selected_component that is not an expanded name shall resolve to denote one of the following:
    1. A component (including a discriminant):
      1. The prefix shall resolve to denote an object or value of some non-array composite type (after any implicit dereference). The selector_name shall resolve to denote a discriminant_specification of the type, or, unless the type is a protected type, a component_declaration of the type. The selected_component denotes the corresponding component of the object or value.
    1. A single entry, an entry family, or a protected subprogram:
      1. The prefix shall resolve to denote an object or value of some task or protected type (after any implicit dereference). The selector_name shall resolve to denote an entry_declaration or subprogram_declaration occurring (implicitly or explicitly) within the visible part of that type. The selected_component denotes the corresponding entry, entry family, or protected subprogram.
  1. An expanded name shall resolve to denote a declaration that occurs immediately within a named declarative region, as follows:
    1. The prefix shall resolve to denote either a package (including the current instance of a generic package, or a rename of a package), or an enclosing named construct.
    2. The selector_name shall resolve to denote a declaration that occurs immediately within the declarative region of the package or enclosing construct (the declaration shall be visible at the place of the expanded name -- see section Visibility.). The expanded name denotes that declaration.
    3. If the prefix does not denote a package, then it shall be a direct_name or an expanded name, and it shall resolve to denote a program unit (other than a package), the current instance of a type, a block_statement, a loop_statement, or an accept_statement (in the case of an accept_statement or entry_body, no family index is allowed); the expanded name shall occur within the declarative region of this construct. Further, if this construct is a callable construct and the prefix denotes more than one such enclosing callable construct, then the expanded name is ambiguous, independently of the selector_name.

Dynamic Semantics

  1. The evaluation of a selected_component includes the evaluation of the prefix.
  2. For a selected_component that denotes a component of a variant, a check is made that the values of the discriminants are such that the value or object denoted by the prefix has this component. The exception Constraint_Error is raised if this check fails.

    Examples

  3. Examples of selected components: 
  4. Tomorrow.Month
--  a record component                    (see section Record Types)

Next_Car.Owner
--  a record component                    (see section Incomplete Type Declarations)

Next_Car.Owner.Age
--  a record component                    (see section Incomplete Type Declarations)
--  the previous two lines involve implicit dereferences

Writer.Unit
--  a record component (a discriminant)   (see section Variant Parts and Discrete Choices)

Min_Cell(H).Value
--  a record component of the result      (see section Subprogram Declarations)
--  of the function call Min_Cell(H)

Control.Seize
--  an entry of a protected object        (see section Protected Units and Protected Objects)

Pool(K).Write
--  an entry of the task Pool(K)          (see section Protected Units and Protected Objects)
  5. Examples of expanded names: 
  6. Key_Manager."<"
--  an operator of the visible part of a package   (see section Private Operations)

Dot_Product.Sum
--  a variable declared in a function body         (see section Subprogram Declarations)

Buffer.Pool
--  a variable declared in a protected unit        (see section Example of Tasking and Synchronization)

Buffer.Read
--  an entry of a protected unit                   (see section Example of Tasking and Synchronization)

Swap.Temp
--  a variable declared in a block statement       (see section Block Statements)

Standard.Boolean
--  the name of a predefined type                  (see section The Package Standard)

Attributes

  1. An attribute is a characteristic of an entity that can be queried via an attribute_reference or a range_attribute_reference.

    Syntax 

  2. attribute_reference ::= prefix'attribute_designator

  3. attribute_designator ::=
     identifier[(static_expression)]
   | Access | Delta | Digits

  4. range_attribute_reference ::= prefix'range_attribute_designator

  5. range_attribute_designator ::= Range[(static_expression)]

    Name Resolution Rules

  6. In an attribute_reference, if the attribute_designator is for an attribute defined for (at least some) objects of an access type, then the prefix is never interpreted as an implicit_dereference; otherwise (and for all range_attribute_references), if the type of the name within the prefix is of an access type, the prefix is interpreted as an implicit_dereference. Similarly, if the attribute_designator is for an attribute defined for (at least some) functions, then the prefix is never interpreted as a parameterless function_call; otherwise (and for all range_attribute_references), if the prefix consists of a name that denotes a function, it is interpreted as a parameterless function_call.
  7. The expression, if any, in an attribute_designator or range_attribute_designator is expected to be of any integer type.

    Legality Rules

  8. The expression, if any, in an attribute_designator or range_attribute_designator shall be static.

    Static Semantics

  9. An attribute_reference denotes a value, an object, a subprogram, or some other kind of program entity.
  10. A range_attribute_reference X'Range(N) is equivalent to the range X'First(N) .. X'Last(N), except that the prefix is only evaluated once. Similarly, X'Range is equivalent to X'First .. X'Last, except that the prefix is only evaluated once.

    Dynamic Semantics

  11. The evaluation of an attribute_reference (or range_attribute_reference) consists of the evaluation of the prefix.

    Implementation Permissions

  12. An implementation may provide implementation-defined attributes; the identifier for an implementation-defined attribute shall differ from those of the language-defined attributes. NOTES
  13. (4) Attributes are defined throughout this International Standard, and are summarized in section Language-Defined Attributes (informative).
  14. (5) In general, the name in a prefix of an attribute_reference (or a range_attribute_reference) has to be resolved without using any context. However, in the case of the Access attribute, the expected type for the prefix has to be a single access type, and if it is an access-to-subprogram type, see section Operations of Access Types, then the resolution of the name can use the fact that the profile of the callable entity denoted by the prefix has to be type conformant with the designated profile of the access type.

    Examples

  15. Examples of attributes: 
  16. Color'First
-- minimum value of the enumeration type Color    (see section Enumeration Types)

Rainbow'Base'First
-- same as Color'First                            (see section Enumeration Types)

Real'Digits
-- precision of the type Real                     (see section Floating Point Types)

Board'Last(2)
-- upper bound of the second dimension of Board   (see section Index Constraints and Discrete Ranges)

Board'Range(1)
-- index range of the first dimension of Board    (see section Index Constraints and Discrete Ranges)

Pool(K)'Terminated
-- True if task Pool(K) is terminated             (see section Task Units and Task Objects)

Date'Size
-- number of bits for records of type Date        (see section Record Types)

Message'Address
-- address of the record variable Message         (see section Discriminant Constraints)

Literals

  1. A literal represents a value literally, that is, by means of notation suited to its kind. A literal is either a numeric_literal, a character_literal, the literal null, or a string_literal.

    Name Resolution Rules

  2. The expected type for a literal null shall be a single access type.
  3. For a name that consists of a character_literal, either its expected type shall be a single character type, in which case it is interpreted as a parameterless function_call that yields the corresponding value of the character type, or its expected profile shall correspond to a parameterless function with a character result type, in which case it is interpreted as the name of the corresponding parameterless function declared as part of the character type's definition, see section Enumeration Types. In either case, the character_literal denotes the enumeration_literal_specification.
  4. The expected type for a primary that is a string_literal shall be a single string type.

    Legality Rules

  5. A character_literal that is a name shall correspond to a defining_character_literal of the expected type, or of the result type of the expected profile.
  6. For each character of a string_literal with a given expected string type, there shall be a corresponding defining_character_literal of the component type of the expected string type.
  7. A literal null shall not be of an anonymous access type, since such types do not have a null value, see section Access Types.

    Static Semantics

  8. An integer literal is of type universal_integer. A real literal is of type universal_real.

    Dynamic Semantics

  9. The evaluation of a numeric literal, or the literal null, yields the represented value.
  10. The evaluation of a string_literal that is a primary yields an array value containing the value of each character of the sequence of characters of the string_literal, as defined in section String Literals. The bounds of this array value are determined according to the rules for positional_array_aggregates (see section Array Aggregates), except that for a null string literal the upper bound is the predecessor of the lower bound.
  11. For the evaluation of a string_literal of type T, a check is made that the value of each character of the string_literal belongs to the component subtype of T. For the evaluation of a null string literal, a check is made that its lower bound is greater than the lower bound of the base range of the index type. The exception Constraint_Error is raised if either of these checks fails. NOTES
  12. (6) Enumeration literals that are identifiers rather than character_literals follow the normal rules for identifiers when used in a name, see section Names, and section Selected Components. Character_literals used as selector_names follow the normal rules for expanded names, see section Selected Components.

    Examples

  13. Examples of literals: 
  14. 3.14159_26536    --  a real literal
1_345            --  an integer literal
'A'              --  a character literal
"Some Text"      --  a string literal

Aggregates

  1. An aggregate combines component values into a composite value of an array type, record type, or record extension.

    Syntax 

  2. aggregate ::=
   record_aggregate | extension_aggregate | array_aggregate

    Name Resolution Rules

  3. The expected type for an aggregate shall be a single nonlimited array type, record type, or record extension.

    Legality Rules

  4. An aggregate shall not be of a class-wide type.

    Dynamic Semantics

  5. For the evaluation of an aggregate, an anonymous object is created and values for the components or ancestor part are obtained (as described in the subsequent subclause for each kind of the aggregate) and assigned into the corresponding components or ancestor part of the anonymous object. Obtaining the values and the assignments occur in an arbitrary order. The value of the aggregate is the value of this object.
  6. If an aggregate is of a tagged type, a check is made that its value belongs to the first subtype of the type. Constraint_Error is raised if this check fails.

Record Aggregates

  1. In a record_aggregate, a value is specified for each component of the record or record extension value, using either a named or a positional association.

    Syntax 

  2. record_aggregate ::= (record_component_association_list)

  3. record_component_association_list ::=
     record_component_association {, record_component_association}
   | null record

  4. record_component_association ::=
   [ component_choice_list => ] expression

  5. component_choice_list ::=
     component_selector_name {| component_selector_name}
   | others
    1. A record_component_association is a named component association if it has a component_choice_list; otherwise, it is a positional component association. Any positional component associations shall precede any named component associations. If there is a named association with a component_choice_list of others, it shall come last.
    2. In the record_component_association_list for a record_aggregate, if there is only one association, it shall be a named association.

Name Resolution Rules

  1. The expected type for a record_aggregate shall be a single nonlimited record type or record extension.
  2. For the record_component_association_list of a record_aggregate, all components of the composite value defined by the aggregate are needed; for the association list of an extension_aggregate, only those components not determined by the ancestor expression or subtype are needed, see section Extension Aggregates. Each selector_name in a record_component_association shall denote a needed component (including possibly a discriminant).
  3. The expected type for the expression of a record_component_association is the type of the associated component(s); the associated component(s) are as follows:
    1. For a positional association, the component (including possibly a discriminant) in the corresponding relative position (in the declarative region of the type), counting only the needed components;
    2. For a named association with one or more component_selector_names, the named component(s);
    3. For a named association with the reserved word others, all needed components that are not associated with some previous association.

Legality Rules

  1. If the type of a record_aggregate is a record extension, then it shall be a descendant of a record type, through one or more record extensions (and no private extensions).
  2. If there are no components needed in a given record_component_association_list, then the reserved words null record shall appear rather than a list of record_component_associations.
  3. Each record_component_association shall have at least one associated component, and each needed component shall be associated with exactly one record_component_association. If a record_component_association has two or more associated components, all of them shall be of the same type.
  4. If the components of a variant_part are needed, then the value of a discriminant that governs the variant_part shall be given by a static expression.

    Dynamic Semantics

  5. The evaluation of a record_aggregate consists of the evaluation of the record_component_association_list.
  6. For the evaluation of a record_component_association_list, any per-object constraints, see section Record Types, for components specified in the association list are elaborated and any expressions are evaluated and converted to the subtype of the associated component. Any constraint elaborations and expression evaluations (and conversions) occur in an arbitrary order, except that the expression for a discriminant is evaluated (and converted) prior to the elaboration of any per-object constraint that depends on it, which in turn occurs prior to the evaluation and conversion of the expression for the component with the per-object constraint.
  7. The expression of a record_component_association is evaluated (and converted) once for each associated component. NOTES
  8. (7) For a record_aggregate with positional associations, expressions specifying discriminant values appear first since the known_discriminant_part is given first in the declaration of the type; they have to be in the same order as in the known_discriminant_part.

    Examples

  9. Example of a record aggregate with positional associations: 
  10. (4, July, 1776)  --  see section Record Types
  11. Examples of record aggregates with named associations: 
  12. (Day => 4, Month => July, Year => 1776)
(Month => July, Day => 4, Year => 1776)

  13. (Disk, Closed, Track => 5, Cylinder => 12)  --  see section Variant Parts and Discrete Choices
(Unit => Disk, Status => Closed, Cylinder => 9, Track => 1)
  14. Example of component association with several choices: 
  15. (Value => 0, Succ|Pred => new Cell'(0, null, null))
--  see section Incomplete Type Declarations

  16. --  The allocator is evaluated twice:
--  Succ and Pred designate different cells
  17. Examples of record aggregates for tagged types, see section Tagged Types and Type Extensions, and section Type Extensions 
  18. Expression'(null record)
Literal'(Value => 0.0)
Painted_Point'(0.0, Pi/2.0, Paint => Red)

Extension Aggregates

  1. An extension_aggregate specifies a value for a type that is a record extension by specifying a value or subtype for an ancestor of the type, followed by associations for any components not determined by the ancestor_part.

    Syntax 

  2. extension_aggregate ::=
   (ancestor_part with record_component_association_list)

  3. ancestor_part ::= expression | subtype_mark

    Name Resolution Rules

  4. The expected type for an extension_aggregate shall be a single nonlimited type that is a record extension. If the ancestor_part is an expression, it is expected to be of any nonlimited tagged type.

    Legality Rules

  5. If the ancestor_part is a subtype_mark, it shall denote a specific tagged subtype. The type of the extension_aggregate shall be derived from the type of the ancestor_part, through one or more record extensions (and no private extensions).

    Static Semantics

  6. For the record_component_association_list of an extension_aggregate, the only components needed are those of the composite value defined by the aggregate that are not inherited from the type of the ancestor_part, plus any inherited discriminants if the ancestor_part is a subtype_mark that denotes an unconstrained subtype.

    Dynamic Semantics

  7. For the evaluation of an extension_aggregate, the record_component_association_list is evaluated. If the ancestor_part is an expression, it is also evaluated; if the ancestor_part is a subtype_mark, the components of the value of the aggregate not given by the record_component_association_list are initialized by default as for an object of the ancestor type. Any implicit initializations or evaluations are performed in an arbitrary order, except that the expression for a discriminant is evaluated prior to any other evaluation or initialization that depends on it.
  8. If the type of the ancestor_part has discriminants that are not inherited by the type of the extension_aggregate, then, unless the ancestor_part is a subtype_mark that denotes an unconstrained subtype, a check is made that each discriminant of the ancestor has the value specified for a corresponding discriminant, either in the record_component_association_list, or in the derived_type_definition for some ancestor of the type of the extension_aggregate. Constraint_Error is raised if this check fails. NOTES
  9. (8) If all components of the value of the extension_aggregate are determined by the ancestor_part, then the record_component_association_list is required to be simply null record.
  10. (9) If the ancestor_part is a subtype_mark, then its type can be abstract. If its type is controlled, then as the last step of evaluating the aggregate, the Initialize procedure of the ancestor type is called, unless the Initialize procedure is abstract, see section User-Defined Assignment and Finalization.

    Examples

  11. Examples of extension aggregates (for types defined in section Type Extensions.): 
  12. Painted_Point'(Point with Red)
(Point'(P) with Paint => Black)

  13. (Expression with Left => 1.2, Right => 3.4)
Addition'(Binop with null record)
-- presuming Binop is of type Binary_Operation

Array Aggregates

  1. In an array_aggregate, a value is specified for each component of an array, either positionally or by its index. For a positional_array_aggregate, the components are given in increasing-index order, with a final others, if any, representing any remaining components. For a named_array_aggregate, the components are identified by the values covered by the discrete_choices.

    Syntax 

  2. array_aggregate ::=
   positional_array_aggregate | named_array_aggregate

  3. positional_array_aggregate ::=
     (expression, expression {, expression})
   | (expression {, expression}, others => expression)

  4. named_array_aggregate ::=
   (array_component_association {, array_component_association})

  5. array_component_association ::=
   discrete_choice_list => expression
  6. An n-dimensional array_aggregate is one that is written as n levels of nested array_aggregates (or at the bottom level, equivalent string_literals). For the multidimensional case (n >= 2) the array_aggregates (or equivalent string_literals) at the n-1 lower levels are called subaggregates of the enclosing n-dimensional array_aggregate. The expressions of the bottom level subaggregates (or of the array_aggregate itself if one-dimensional) are called the array component expressions of the enclosing n-dimensional array_aggregate.

    Name Resolution Rules

  7. The expected type for an array_aggregate (that is not a subaggregate) shall be a single nonlimited array type. The component type of this array type is the expected type for each array component expression of the array_aggregate.
  8. The expected type for each discrete_choice in any discrete_choice_list of a named_array_aggregate is the type of the corresponding index; the corresponding index for an array_aggregate that is not a subaggregate is the first index of its type; for an (n-m)-dimensional subaggregate within an array_aggregate of an n-dimensional type, the corresponding index is the index in position m+1.

    Legality Rules

  9. An array_aggregate of an n-dimensional array type shall be written as an n-dimensional array_aggregate.
  10. An others choice is allowed for an array_aggregate only if an applicable index constraint applies to the array_aggregate. An applicable index constraint is a constraint provided by certain contexts where an array_aggregate is permitted that can be used to determine the bounds of the array value specified by the aggregate. Each of the following contexts (and none other) defines an applicable index constraint:
    1. For an explicit_actual_parameter, an explicit_generic_actual_parameter, the expression of a return_statement, the initialization expression in an object_declaration, or a default_expression (for a parameter or a component), when the nominal subtype of the corresponding formal parameter, generic formal parameter, function result, object, or component is a constrained array subtype, the applicable index constraint is the constraint of the subtype;
    2. For the expression of an assignment_statement where the name denotes an array variable, the applicable index constraint is the constraint of the array variable;
    3. For the operand of a qualified_expression whose subtype_mark denotes a constrained array subtype, the applicable index constraint is the constraint of the subtype;
    4. For a component expression in an aggregate, if the component's nominal subtype is a constrained array subtype, the applicable index constraint is the constraint of the subtype;
    5. For a parenthesized expression, the applicable index constraint is that, if any, defined for the expression.
  1. The applicable index constraint applies to an array_aggregate that appears in such a context, as well as to any subaggregates thereof. In the case of an explicit_actual_parameter (or default_expression) for a call on a generic formal subprogram, no applicable index constraint is defined.
  2. The discrete_choice_list of an array_component_association is allowed to have a discrete_choice that is a nonstatic expression or that is a discrete_range that defines a nonstatic or null range, only if it is the single discrete_choice of its discrete_choice_list, and there is only one array_component_association in the array_aggregate.
  3. In a named_array_aggregate with more than one discrete_choice, no two discrete_choices are allowed to cover the same value, see section Variant Parts and Discrete Choices, if there is no others choice, the discrete_choices taken together shall exactly cover a contiguous sequence of values of the corresponding index type.
  4. A bottom level subaggregate of a multidimensional array_aggregate of a given array type is allowed to be a string_literal only if the component type of the array type is a character type; each character of such a string_literal shall correspond to a defining_character_literal of the component type.

    Static Semantics

  5. A subaggregate that is a string_literal is equivalent to one that is a positional_array_aggregate of the same length, with each expression being the character_literal for the corresponding character of the string_literal.

    Dynamic Semantics

  6. The evaluation of an array_aggregate of a given array type proceeds in two steps:
    1. Any discrete_choices of this aggregate and of its subaggregates are evaluated in an arbitrary order, and converted to the corresponding index type;
    2. The array component expressions of the aggregate are evaluated in an arbitrary order and their values are converted to the component subtype of the array type; an array component expression is evaluated once for each associated component.
  1. The bounds of the index range of an array_aggregate (including a subaggregate) are determined as follows:
    1. For an array_aggregate with an others choice, the bounds are those of the corresponding index range from the applicable index constraint;
    2. For a positional_array_aggregate (or equivalent string_literal) without an others choice, the lower bound is that of the corresponding index range in the applicable index constraint, if defined, or that of the corresponding index subtype, if not; in either case, the upper bound is determined from the lower bound and the number of expressions (or the length of the string_literal);
    3. For a named_array_aggregate without an others choice, the bounds are determined by the smallest and largest index values covered by any discrete_choice_list.
  1. For an array_aggregate, a check is made that the index range defined by its bounds is compatible with the corresponding index subtype.
  2. For an array_aggregate with an others choice, a check is made that no expression is specified for an index value outside the bounds determined by the applicable index constraint.
  3. For a multidimensional array_aggregate, a check is made that all subaggregates that correspond to the same index have the same bounds.
  4. The exception Constraint_Error is raised if any of the above checks fail. NOTES
  5. (10) In an array_aggregate, positional notation may only be used with two or more expressions; a single expression in parentheses is interpreted as a parenthesized_expression. A named_array_aggregate, such as (1 => X), may be used to specify an array with a single component.

    Examples

  6. Examples of array aggregates with positional associations: 
  7. (7, 9, 5, 1, 3, 2, 4, 8, 6, 0)
Table'(5, 8, 4, 1, others => 0)  --  see section Array Types
  8. Examples of array aggregates with named associations: 
  9. (1 .. 5 => (1 .. 8 => 0.0))  --  two-dimensional
(1 .. N => new Cell)         --  N new cells, in particular for N = 0

  10. Table'(2 | 4 | 10 => 1, others => 0)

Schedule'(Mon .. Fri => True,  others => False)
Schedule'(Wed | Sun  => False, others => True)
--  see section Array Types

Vector'(1 => 2.5)
--  single-component vector
  11. Examples of two-dimensional array aggregates: 
  12. -- Three aggregates for the same value of subtype
--  Matrix(1..2,1..3), see section Array Types

  13. ((1.1, 1.2, 1.3), (2.1, 2.2, 2.3))
(1 => (1.1, 1.2, 1.3), 2 => (2.1, 2.2, 2.3))
(1 => (1 => 1.1, 2 => 1.2, 3 => 1.3),
  2 => (1 => 2.1, 2 => 2.2, 3 => 2.3))
  14. Examples of aggregates as initial values: 
  15. A : Table := (7, 9, 5, 1, 3, 2, 4, 8, 6, 0);
-- A(1)=7, A(10)=0

B : Table := (2 | 4 | 10 => 1, others => 0);
-- B(1)=0, B(10)=1

C : constant Matrix := (1 .. 5 => (1 .. 8 => 0.0));
-- C'Last(1)=5, C'Last(2)=8

  16. D : Bit_Vector(M .. N) := (M .. N => True);  --  see section Array Types
E : Bit_Vector(M .. N) := (others => True);
F : String(1 .. 1) := (1 => 'F');
-- a one component aggregate: same as "F"

Expressions

  1. An expression is a formula that defines the computation or retrieval of a value. In this International Standard, the term "expression" refers to a construct of the syntactic category expression or of any of the other five syntactic categories defined below.

    Syntax 

  2. expression ::=
     relation {and relation} | relation {and then relation}
   | relation {or relation}  | relation {or else relation}
   | relation {xor relation}

  3. relation ::=
     simple_expression [relational_operator simple_expression]
   | simple_expression [not] in range
   | simple_expression [not] in subtype_mark

  4. simple_expression ::=
   [unary_adding_operator] term {binary_adding_operator term}

  5. term ::= factor {multiplying_operator factor}

  6. factor ::= primary [** primary] | abs primary | not primary

  7. primary ::=
     numeric_literal   | null
   | string_literal    | aggregate
   | name              | qualified_expression
   | allocator         | (expression)

    Name Resolution Rules

  8. A name used as a primary shall resolve to denote an object or a value.

    Static Semantics

  9. Each expression has a type; it specifies the computation or retrieval of a value of that type.

    Dynamic Semantics

  10. The value of a primary that is a name denoting an object is the value of the object.

    Implementation Permissions

  11. For the evaluation of a primary that is a name denoting an object of an unconstrained numeric subtype, if the value of the object is outside the base range of its type, the implementation may either raise Constraint_Error or return the value of the object.

    Examples

  12. Examples of primaries: 
  13. 4.0                --  real literal
Pi                 --  named number
(1 .. 10 => 0)     --  array aggregate
Sum                --  variable
Integer'Last       --  attribute
Sine(X)            --  function call
Color'(Blue)       --  qualified expression
Real(M*N)          --  conversion
(Line_Count + 10)  --  parenthesized expression
  14. Examples of expressions: 
  15. Volume                      -- primary
not Destroyed               -- factor
2*Line_Count                -- term
-4.0                        -- simple expression
-4.0 + A                    -- simple expression
B**2 - 4.0*A*C              -- simple expression
Password(1 .. 3) = "Bwv"    -- relation
Count in Small_Int          -- relation
Count not in Small_Int      -- relation
Index = 0 or Item_Hit       -- expression
(Cold and Sunny) or Warm    -- expression (parentheses are required)
A**(B**C)                   -- expression (parentheses are required)

Operators and Expression Evaluation

  1. The language defines the following six categories of operators (given in order of increasing precedence). The corresponding operator_symbols, and only those, can be used as designators in declarations of functions for user-defined operators (see section Overloading of Operators).

    Syntax 

  2. logical_operator            ::=  and | or  | xor

  3. relational_operator         ::=  =   | /=  | <   | <= | > | >=

  4. binary_adding_operator      ::=  +   | -   | &

  5. unary_adding_operator       ::=  +   | -

  6. multiplying_operator        ::=  *   | /   | mod | rem

  7. highest_precedence_operator ::=  **  | abs | not

    Static Semantics

  8. For a sequence of operators of the same precedence level, the operators are associated with their operands in textual order from left to right. Parentheses can be used to impose specific associations.
  9. For each form of type definition, certain of the above operators are predefined; that is, they are implicitly declared immediately after the type definition. For each such implicit operator declaration, the parameters are called Left and Right for binary operators; the single parameter is called Right for unary operators. An expression of the form X op Y, where op is a binary operator, is equivalent to a function_call of the form "op"(X, Y). An expression of the form op Y, where op is a unary operator, is equivalent to a function_call of the form "op"(Y). The predefined operators and their effects are described in subclauses section Logical Operators and Short-circuit Control Forms through section Highest Precedence Operators.

    Dynamic Semantics

  10. The predefined operations on integer types either yield the mathematically correct result or raise the exception Constraint_Error. For implementations that support the Numerics Annex, the predefined operations on real types yield results whose accuracy is defined in section Numerics (normative), or raise the exception Constraint_Error.

    Implementation Requirements

  11. The implementation of a predefined operator that delivers a result of an integer or fixed point type may raise Constraint_Error only if the result is outside the base range of the result type.
  12. The implementation of a predefined operator that delivers a result of a floating point type may raise Constraint_Error only if the result is outside the safe range of the result type.

    Implementation Permissions

  13. For a sequence of predefined operators of the same precedence level (and in the absence of parentheses imposing a specific association), an implementation may impose any association of the operators with operands so long as the result produced is an allowed result for the left-to-right association, but ignoring the potential for failure of language-defined checks in either the left-to-right or chosen order of association. NOTES
  14. (11) The two operands of an expression of the form X op Y, where op is a binary operator, are evaluated in an arbitrary order, as for any function_call, see section Subprogram Calls.

    Examples

  15. Examples of precedence: 
  16. not Sunny or Warm    --  same as (not Sunny) or Warm
X > 4.0 and Y > 0.0  --  same as (X > 4.0) and (Y > 0.0)

  17. -4.0*A**2            --  same as -(4.0 * (A**2))
abs(1 + A) + B       --  same as (abs (1 + A)) + B
Y**(-3)              --  parentheses are necessary
A / B * C            --  same as (A/B)*C
A + (B + C)          --  evaluate B + C before adding it to A

Logical Operators and Short-circuit Control Forms

Name Resolution Rules

  1. An expression consisting of two relations connected by and then or or else (a short-circuit control form) shall resolve to be of some boolean type; the expected type for both relations is that same boolean type.

    Static Semantics

  2. The following logical operators are predefined for every boolean type T, for every modular type T, and for every one-dimensional array type T whose component type is a boolean type: 
  3. function "and"(Left, Right : T) return T
function "or" (Left, Right : T) return T
function "xor"(Left, Right : T) return T
  4. For boolean types, the predefined logical operators and, or, and xor perform the conventional operations of conjunction, inclusive disjunction, and exclusive disjunction, respectively.
  5. For modular types, the predefined logical operators are defined on a bit-by-bit basis, using the binary representation of the value of the operands to yield a binary representation for the result, where zero represents False and one represents True. If this result is outside the base range of the type, a final subtraction by the modulus is performed to bring the result into the base range of the type.
  6. The logical operators on arrays are performed on a component-by-component basis on matching components (as for equality -- see section Relational Operators and Membership Tests.), using the predefined logical operator for the component type. The bounds of the resulting array are those of the left operand.

    Dynamic Semantics

  7. The short-circuit control forms and then and or else deliver the same result as the corresponding predefined and and or operators for boolean types, except that the left operand is always evaluated first, and the right operand is not evaluated if the value of the left operand determines the result.
  8. For the logical operators on arrays, a check is made that for each component of the left operand there is a matching component of the right operand, and vice versa. Also, a check is made that each component of the result belongs to the component subtype. The exception Constraint_Error is raised if either of the above checks fails. NOTES
  9. (12) The conventional meaning of the logical operators is given by the following truth table: 
  10. A      B      (A and B)  (A or B)  (A xor B)

True   True     True       True      False
True   False    False      True      True
False  True     False      True      True
False  False    False      False     False

    Examples

  11. Examples of logical operators: 
  12. Sunny or Warm
Filter(1 .. 10) and Filter(15 .. 24)   --  see section Index Constraints and Discrete Ranges
  13. Examples of short-circuit control forms: 
  14. Next_Car.Owner /= null and then Next_Car.Owner.Age > 25
--  see section Incomplete Type Declarations

N = 0 or else A(N) = Hit_Value

Relational Operators and Membership Tests

  1. The equality operators = (equals) and /= (not equals) are predefined for nonlimited types. The other relational_operators are the ordering operators < (less than), <= (less than or equal), > (greater than), and >= (greater than or equal). The ordering operators are predefined for scalar types, and for discrete array types, that is, one-dimensional array types whose components are of a discrete type.
  2. A membership test, using in or not in, determines whether or not a value belongs to a given subtype or range, or has a tag that identifies a type that is covered by a given type. Membership tests are allowed for all types.

    Name Resolution Rules

  3. The tested type of a membership test is the type of the range or the type determined by the subtype_mark. If the tested type is tagged, then the simple_expression shall resolve to be of a type that covers or is covered by the tested type; if untagged, the expected type for the simple_expression is the tested type.

    Legality Rules

  4. For a membership test, if the simple_expression is of a tagged class-wide type, then the tested type shall be (visibly) tagged.

    Static Semantics

  5. The result type of a membership test is the predefined type Boolean.
  6. The equality operators are predefined for every specific type T that is not limited, and not an anonymous access type, with the following specifications: 
  7. function "=" (Left, Right : T) return Boolean
function "/="(Left, Right : T) return Boolean
  8. The ordering operators are predefined for every specific scalar type T, and for every discrete array type T, with the following specifications: 
  9. function "<" (Left, Right : T) return Boolean
function "<="(Left, Right : T) return Boolean
function ">" (Left, Right : T) return Boolean
function ">="(Left, Right : T) return Boolean

    Dynamic Semantics

  10. For discrete types, the predefined relational operators are defined in terms of corresponding mathematical operations on the position numbers of the values of the operands.
  11. For real types, the predefined relational operators are defined in terms of the corresponding mathematical operations on the values of the operands, subject to the accuracy of the type.
  12. Two access-to-object values are equal if they designate the same object, or if both are equal to the null value of the access type.
  13. Two access-to-subprogram values are equal if they are the result of the same evaluation of an Access attribute_reference, or if both are equal to the null value of the access type. Two access-to-subprogram values are unequal if they designate different subprograms. It is unspecified whether two access values that designate the same subprogram but are the result of distinct evaluations of Access attribute_references are equal or unequal.
  14. For a type extension, predefined equality is defined in terms of the primitive (possibly user-defined) equals operator of the parent type and of any tagged components of the extension part, and predefined equality for any other components not inherited from the parent type.
  15. For a private type, if its full type is tagged, predefined equality is defined in terms of the primitive equals operator of the full type; if the full type is untagged, predefined equality for the private type is that of its full type.
  16. For other composite types, the predefined equality operators (and certain other predefined operations on composite types -- see section Logical Operators and Short-circuit Control Forms, and section Type Conversions.) are defined in terms of the corresponding operation on matching components, defined as follows:
    1. For two composite objects or values of the same non-array type, matching components are those that correspond to the same component_declaration or discriminant_specification;
    2. For two one-dimensional arrays of the same type, matching components are those (if any) whose index values match in the following sense: the lower bounds of the index ranges are defined to match, and the successors of matching indices are defined to match;
    3. For two multidimensional arrays of the same type, matching components are those whose index values match in successive index positions.
  1. The analogous definitions apply if the types of the two objects or values are convertible, rather than being the same.
  2. Given the above definition of matching components, the result of the predefined equals operator for composite types (other than for those composite types covered earlier) is defined as follows:
    1. If there are no components, the result is defined to be True;
    2. If there are unmatched components, the result is defined to be False;
    3. Otherwise, the result is defined in terms of the primitive equals operator for any matching tagged components, and the predefined equals for any matching untagged components.
  1. The predefined "/=" operator gives the complementary result to the predefined "=" operator.
  2. For a discrete array type, the predefined ordering operators correspond to lexicographic order using the predefined order relation of the component type: A null array is lexicographically less than any array having at least one component. In the case of nonnull arrays, the left operand is lexicographically less than the right operand if the first component of the left operand is less than that of the right; otherwise the left operand is lexicographically less than the right operand only if their first components are equal and the tail of the left operand is lexicographically less than that of the right (the tail consists of the remaining components beyond the first and can be null).
  3. For the evaluation of a membership test, the simple_expression and the range (if any) are evaluated in an arbitrary order.
  4. A membership test using in yields the result True if:
    1. The tested type is scalar, and the value of the simple_expression belongs to the given range, or the range of the named subtype; or
    2. The tested type is not scalar, and the value of the simple_ expression satisfies any constraints of the named subtype, and, if the type of the simple_expression is class-wide, the value has a tag that identifies a type covered by the tested type.
  1. Otherwise the test yields the result False.
  2. A membership test using not in gives the complementary result to the corresponding membership test using in. NOTES
  3. (13) No exception is ever raised by a membership test, by a predefined ordering operator, or by a predefined equality operator for an elementary type, but an exception can be raised by the evaluation of the operands. A predefined equality operator for a composite type can only raise an exception if the type has a tagged part whose primitive equals operator propagates an exception.
  4. (14) If a composite type has components that depend on discriminants, two values of this type have matching components if and only if their discriminants are equal. Two nonnull arrays have matching components if and only if the length of each dimension is the same for both.

    Examples

  5. Examples of expressions involving relational operators and membership tests: 
  6. X /= Y

  7. "" < "A" and "A" < "Aa"     --  True
"Aa" < "B" and "A" < "A  "  --  True

  8. My_Car = null
-- true if My_Car has been set to null  (see section Incomplete Type Declarations)

My_Car = Your_Car
-- true if we both share the same car

My_Car.all = Your_Car.all
-- true if the two cars are identical

  9. N not in 1 .. 10
-- range membership test

Today in Mon .. Fri
-- range membership test

Today in Weekday
-- subtype membership test  (see section Enumeration Types)

Archive in Disk_Unit
-- subtype membership test, see section Variant Parts and Discrete Choices

Tree.all in Addition'Class
-- class membership test  (see section Type Extensions)

Binary Adding Operators

Static Semantics

  1. The binary adding operators + (addition) and - (subtraction) are predefined for every specific numeric type T with their conventional meaning. They have the following specifications: 
  2. function "+"(Left, Right : T) return T
function "-"(Left, Right : T) return T
  3. The concatenation operators & are predefined for every nonlimited, one-dimensional array type T with component type C. They have the following specifications: 
  4. function "&"(Left : T; Right : T) return T
function "&"(Left : T; Right : C) return T
function "&"(Left : C; Right : T) return T
function "&"(Left : C; Right : C) return T

    Dynamic Semantics

  5. For the evaluation of a concatenation with result type T, if both operands are of type T, the result of the concatenation is a one-dimensional array whose length is the sum of the lengths of its operands, and whose components comprise the components of the left operand followed by the components of the right operand. If the left operand is a null array, the result of the concatenation is the right operand. Otherwise, the lower bound of the result is determined as follows:
    1. If the ultimate ancestor of the array type was defined by a constrained_array_definition, then the lower bound of the result is that of the index subtype;
    2. If the ultimate ancestor of the array type was defined by an unconstrained_array_definition, then the lower bound of the result is that of the left operand.
  1. The upper bound is determined by the lower bound and the length. A check is made that the upper bound of the result of the concatenation belongs to the range of the index subtype, unless the result is a null array. Constraint_Error is raised if this check fails.
  2. If either operand is of the component type C, the result of the concatenation is given by the above rules, using in place of such an operand an array having this operand as its only component (converted to the component subtype) and having the lower bound of the index subtype of the array type as its lower bound.
  3. The result of a concatenation is defined in terms of an assignment to an anonymous object, as for any function call, see section Return Statements. NOTES
  4. (15) As for all predefined operators on modular types, the binary adding operators + and - on modular types include a final reduction modulo the modulus if the result is outside the base range of the type.

    Examples

  5. Examples of expressions involving binary adding operators: 
  6. Z + 0.1
--  Z has to be of a real type

  7. "A" & "BCD"
--  concatenation of two string literals

'A' & "BCD"
--  concatenation of a character literal and a string literal

'A' & 'A'
--  concatenation of two character literals

Unary Adding Operators

Static Semantics

  1. The unary adding operators + (identity) and - (negation) are predefined for every specific numeric type T with their conventional meaning. They have the following specifications: 
  2. function "+"(Right : T) return T
function "-"(Right : T) return T
    NOTES
  3. (16) For modular integer types, the unary adding operator -, when given a nonzero operand, returns the result of subtracting the value of the operand from the modulus; for a zero operand, the result is zero.

Multiplying Operators

Static Semantics

  1. The multiplying operators * (multiplication), / (division), mod (modulus), and rem (remainder) are predefined for every specific integer type T: 
  2. function "*"  (Left, Right : T) return T
function "/"  (Left, Right : T) return T
function "mod"(Left, Right : T) return T
function "rem"(Left, Right : T) return T
  3. Signed integer multiplication has its conventional meaning.
  4. Signed integer division and remainder are defined by the relation: 
  5. A = (A/B)*B + (A rem B)
  6. where (A rem B) has the sign of A and an absolute value less than the absolute value of B. Signed integer division satisfies the identity: 
  7. (-A)/B = -(A/B) = A/(-B)
  8. The signed integer modulus operator is defined such that the result of A mod B has the sign of B and an absolute value less than the absolute value of B; in addition, for some signed integer value N, this result satisfies the relation: 
  9. A = B*N + (A mod B)
  10. The multiplying operators on modular types are defined in terms of the corresponding signed integer operators, followed by a reduction modulo the modulus if the result is outside the base range of the type (which is only possible for the "*" operator).
  11. Multiplication and division operators are predefined for every specific floating point type T: 
  12. function "*"(Left, Right : T) return T
function "/"(Left, Right : T) return T
  13. The following multiplication and division operators, with an operand of the predefined type Integer, are predefined for every specific fixed point type T: 
  14. function "*"(Left : T; Right : Integer) return T
function "*"(Left : Integer; Right : T) return T
function "/"(Left : T; Right : Integer) return T
  15. All of the above multiplying operators are usable with an operand of an appropriate universal numeric type. The following additional multiplying operators for root_real are predefined, and are usable when both operands are of an appropriate universal or root numeric type, and the result is allowed to be of type root_real, as in a number_declaration: 
  16. function "*"(Left, Right : root_real) return root_real
function "/"(Left, Right : root_real) return root_real

  17. function "*"(Left : root_real; Right : root_integer) return root_real
function "*"(Left : root_integer; Right : root_real) return root_real
function "/"(Left : root_real; Right : root_integer) return root_real
  18. Multiplication and division between any two fixed point types are provided by the following two predefined operators: 
  19. function "*"(Left, Right : universal_fixed) return universal_fixed
function "/"(Left, Right : universal_fixed) return universal_fixed

    Legality Rules

  20. The above two fixed-fixed multiplying operators shall not be used in a context where the expected type for the result is itself universal_fixed -- the context has to identify some other numeric type to which the result is to be converted, either explicitly or implicitly.

    Dynamic Semantics

  21. The multiplication and division operators for real types have their conventional meaning. For floating point types, the accuracy of the result is determined by the precision of the result type. For decimal fixed point types, the result is truncated toward zero if the mathematical result is between two multiples of the small of the specific result type (possibly determined by context); for ordinary fixed point types, if the mathematical result is between two multiples of the small, it is unspecified which of the two is the result.
  22. The exception Constraint_Error is raised by integer division, rem, and mod if the right operand is zero. Similarly, for a real type T with T'Machine_Overflows True, division by zero raises Constraint_Error. NOTES
  23. (17) For positive A and B, A/B is the quotient and A rem B is the remainder when A is divided by B. The following relations are satisfied by the rem operator: 
  24.   A  rem (-B) =   A rem B
(-A) rem   B  = -(A rem B)
  25. (18) For any signed integer K, the following identity holds: 
  26.   A mod B   =   (A + K*B) mod B
  27. The relations between signed integer division, remainder, and modulus are illustrated by the following table: 
  28.   A    B  A/B  A rem B  A mod B    A   B  A/B  A rem B  A mod B

  29.   10   5   2      0        0      -10  5   -2     0        0
  11   5   2      1        1      -11  5   -2    -1        4
  12   5   2      2        2      -12  5   -2    -2        3
  13   5   2      3        3      -13  5   -2    -3        2
  14   5   2      4        4      -14  5   -2    -4        1

  30.   A    B  A/B  A rem B  A mod B    A   B  A/B  A rem B  A mod B
  10  -5   -2     0        0      -10 -5    2     0        0
  11  -5   -2     1       -4      -11 -5    2    -1       -1
  12  -5   -2     2       -3      -12 -5    2    -2       -2
  13  -5   -2     3       -2      -13 -5    2    -3       -3
  14  -5   -2     4       -1      -14 -5    2    -4       -4

    Examples

  31. Examples of expressions involving multiplying operators: 
  32. I : Integer := 1;
J : Integer := 2;
K : Integer := 3;

  33. X : Real := 1.0;       --  see section Floating Point Types
Y : Real := 2.0;

  34. F : Fraction := 0.25;  --  see section Fixed Point Types
G : Fraction := 0.5;

  35. Expression     Value  Result Type

I*J            2      same as I and J, that is, Integer
K/J            1      same as K and J, that is, Integer
K mod J        1      same as K and J, that is, Integer

X/Y            0.5    same as X and Y, that is, Real
F/2            0.125  same as F, that is, Fraction

3*F            0.75   same as F, that is, Fraction
0.75*G         0.375  universal_fixed, implicitly convertible
                      to any fixed point type
Fraction(F*G)  0.125  Fraction, as stated by the conversion
Real(J)*Y      4.0    Real, the type of both operands after
                      conversion of J

Highest Precedence Operators

Static Semantics

  1. The highest precedence unary operator abs (absolute value) is predefined for every specific numeric type T, with the following specification: 
  2. function "abs"(Right : T) return T
  3. The highest precedence unary operator not (logical negation) is predefined for every boolean type T, every modular type T, and for every one-dimensional array type T whose components are of a boolean type, with the following specification: 
  4. function "not"(Right : T) return T
  5. The result of the operator not for a modular type is defined as the difference between the high bound of the base range of the type and the value of the operand. For a binary modulus, this corresponds to a bit-wise complement of the binary representation of the value of the operand.
  6. The operator not that applies to a one-dimensional array of boolean components yields a one-dimensional boolean array with the same bounds; each component of the result is obtained by logical negation of the corresponding component of the operand (that is, the component that has the same index value). A check is made that each component of the result belongs to the component subtype; the exception Constraint_Error is raised if this check fails.
  7. The highest precedence exponentiation operator ** is predefined for every specific integer type T with the following specification: 
  8. function "**"(Left : T; Right : Natural) return T
  9. Exponentiation is also predefined for every specific floating point type as well as root_real, with the following specification (where T is root_real or the floating point type): 
  10. function "**"(Left : T; Right : Integer'Base) return T
  11. The right operand of an exponentiation is the exponent. The expression X**N with the value of the exponent N positive is equivalent to the expression X*X*...X (with N-1 multiplications) except that the multiplications are associated in an arbitrary order. With N equal to zero, the result is one. With the value of N negative (only defined for a floating point operand), the result is the reciprocal of the result using the absolute value of N as the exponent.

    Implementation Permissions

  12. The implementation of exponentiation for the case of a negative exponent is allowed to raise Constraint_Error if the intermediate result of the repeated multiplications is outside the safe range of the type, even though the final result (after taking the reciprocal) would not be. (The best machine approximation to the final result in this case would generally be 0.0.) NOTES
  13. (19) As implied by the specification given above for exponentiation of an integer type, a check is made that the exponent is not negative. Constraint_Error is raised if this check fails.

Type Conversions

  1. Explicit type conversions, both value conversions and view conversions, are allowed between closely related types as defined below. This clause also defines rules for value and view conversions to a particular subtype of a type, both explicit ones and those implicit in other constructs.

    Syntax 

  2. type_conversion ::=
     subtype_mark(expression)
   | subtype_mark(name)
  3. The target subtype of a type_conversion is the subtype denoted by the subtype_mark. The operand of a type_conversion is the expression or name within the parentheses; its type is the operand type.
  4. One type is convertible to a second type if a type_conversion with the first type as operand type and the second type as target type is legal according to the rules of this clause. Two types are convertible if each is convertible to the other.
  5. A type_conversion whose operand is the name of an object is called a view conversion if its target type is tagged, or if it appears as an actual parameter of mode out or in out; other type_conversions are called value conversions.

    Name Resolution Rules

  6. The operand of a type_conversion is expected to be of any type.
  7. The operand of a view conversion is interpreted only as a name; the operand of a value conversion is interpreted as an expression.

    Legality Rules

  8. If the target type is a numeric type, then the operand type shall be a numeric type.
  9. If the target type is an array type, then the operand type shall be an array type. Further:
    1. The types shall have the same dimensionality;
    2. Corresponding index types shall be convertible; and
    3. The component subtypes shall statically match.
  1. If the target type is a general access type, then the operand type shall be an access-to-object type. Further:
    1. If the target type is an access-to-variable type, then the operand type shall be an access-to-variable type;
    2. If the target designated type is tagged, then the operand designated type shall be convertible to the target designated type;
    3. If the target designated type is not tagged, then the designated types shall be the same, and either the designated subtypes shall statically match or the target designated subtype shall be discriminated and unconstrained; and
    4. The accessibility level of the operand type shall not be statically deeper than that of the target type. In addition to the places where Legality Rules normally apply, see section Generic Instantiation, this rule applies also in the private part of an instance of a generic unit.
  1. If the target type is an access-to-subprogram type, then the operand type shall be an access-to-subprogram type. Further:
    1. The designated profiles shall be subtype-conformant.
    2. The accessibility level of the operand type shall not be statically deeper than that of the target type. In addition to the places where Legality Rules normally apply, see section Generic Instantiation, this rule applies also in the private part of an instance of a generic unit. If the operand type is declared within a generic body, the target type shall be declared within the generic body.
  1. If the target type is not included in any of the above four cases, there shall be a type that is an ancestor of both the target type and the operand type. Further, if the target type is tagged, then either:
    1. The operand type shall be covered by or descended from the target type; or
    2. The operand type shall be a class-wide type that covers the target type.
  1. In a view conversion for an untagged type, the target type shall be convertible (back) to the operand type.

    Static Semantics

  2. A type_conversion that is a value conversion denotes the value that is the result of converting the value of the operand to the target subtype.
  3. A type_conversion that is a view conversion denotes a view of the object denoted by the operand. This view is a variable of the target type if the operand denotes a variable; otherwise it is a constant of the target type.
  4. The nominal subtype of a type_conversion is its target subtype.

    Dynamic Semantics

  5. For the evaluation of a type_conversion that is a value conversion, the operand is evaluated, and then the value of the operand is converted to a corresponding value of the target type, if any. If there is no value of the target type that corresponds to the operand value, Constraint_Error is raised; this can only happen on conversion to a modular type, and only when the operand value is outside the base range of the modular type. Additional rules follow:
    1. Numeric Type Conversion
      1. If the target and the operand types are both integer types, then the result is the value of the target type that corresponds to the same mathematical integer as the operand.
      2. If the target type is a decimal fixed point type, then the result is truncated (toward 0) if the value of the operand is not a multiple of the small of the target type.
      3. If the target type is some other real type, then the result is within the accuracy of the target type (see section Numeric Performance Requirements, for implementations that support the Numerics Annex).
      4. If the target type is an integer type and the operand type is real, the result is rounded to the nearest integer (away from zero if exactly halfway between two integers).
    1. Enumeration Type Conversion
      1. The result is the value of the target type with the same position number as that of the operand value.
    1. Array Type Conversion
      1. If the target subtype is a constrained array subtype, then a check is made that the length of each dimension of the value of the operand equals the length of the corresponding dimension of the target subtype. The bounds of the result are those of the target subtype.
      2. If the target subtype is an unconstrained array subtype, then the bounds of the result are obtained by converting each bound of the value of the operand to the corresponding index type of the target type. For each nonnull index range, a check is made that the bounds of the range belong to the corresponding index subtype.
      3. In either array case, the value of each component of the result is that of the matching component of the operand value, see section Relational Operators and Membership Tests.
    1. Composite (Non-Array) Type Conversion
      1. The value of each nondiscriminant component of the result is that of the matching component of the operand value.
      2. The tag of the result is that of the operand. If the operand type is class-wide, a check is made that the tag of the operand identifies a (specific) type that is covered by or descended from the target type.
      3. For each discriminant of the target type that corresponds to a discriminant of the operand type, its value is that of the corresponding discriminant of the operand value; if it corresponds to more than one discriminant of the operand type, a check is made that all these discriminants are equal in the operand value.
      4. For each discriminant of the target type that corresponds to a discriminant that is specified by the derived_type_definition for some ancestor of the operand type (or if class-wide, some ancestor of the specific type identified by the tag of the operand), its value in the result is that specified by the derived_type_definition.
      5. For each discriminant of the operand type that corresponds to a discriminant that is specified by the derived_type_definition for some ancestor of the target type, a check is made that in the operand value it equals the value specified for it.
      6. For each discriminant of the result, a check is made that its value belongs to its subtype.
    1. Access Type Conversion
      1. For an access-to-object type, a check is made that the accessibility level of the operand type is not deeper than that of the target type.
      2. If the target type is an anonymous access type, a check is made that the value of the operand is not null; if the target is not an anonymous access type, then the result is null if the operand value is null.
      3. If the operand value is not null, then the result designates the same object (or subprogram) as is designated by the operand value, but viewed as being of the target designated subtype (or profile); any checks associated with evaluating a conversion to the target designated subtype are performed.
  1. After conversion of the value to the target type, if the target subtype is constrained, a check is performed that the value satisfies this constraint.
  2. For the evaluation of a view conversion, the operand name is evaluated, and a new view of the object denoted by the operand is created, whose type is the target type; if the target type is composite, checks are performed as above for a value conversion.
  3. The properties of this new view are as follows:
    1. If the target type is composite, the bounds or discriminants (if any) of the view are as defined above for a value conversion; each nondiscriminant component of the view denotes the matching component of the operand object; the subtype of the view is constrained if either the target subtype or the operand object is constrained, or if the operand type is a descendant of the target type, and has discriminants that were not inherited from the target type;
    2. If the target type is tagged, then an assignment to the view assigns to the corresponding part of the object denoted by the operand; otherwise, an assignment to the view assigns to the object, after converting the assigned value to the subtype of the object (which might raise Constraint_Error);
    3. Reading the value of the view yields the result of converting the value of the operand object to the target subtype (which might raise Constraint_Error), except if the object is of an access type and the view conversion is passed as an out parameter; in this latter case, the value of the operand object is used to initialize the formal parameter without checking against any constraint of the target subtype (see section Parameter Associations).
  1. If an Accessibility_Check fails, Program_Error is raised. Any other check associated with a conversion raises Constraint_Error if it fails.
  2. Conversion to a type is the same as conversion to an unconstrained subtype of the type. NOTES
  3. (20) In addition to explicit type_conversions, type conversions are performed implicitly in situations where the expected type and the actual type of a construct differ, as is permitted by the type resolution rules, see section The Context of Overload Resolution. For example, an integer literal is of the type universal_integer, and is implicitly converted when assigned to a target of some specific integer type. Similarly, an actual parameter of a specific tagged type is implicitly converted when the corresponding formal parameter is of a class-wide type.
  4. Even when the expected and actual types are the same, implicit subtype conversions are performed to adjust the array bounds (if any) of an operand to match the desired target subtype, or to raise Constraint_Error if the (possibly adjusted) value does not satisfy the constraints of the target subtype.
  5. (21) A ramification of the overload resolution rules is that the operand of an (explicit) type_conversion cannot be the literal null, an allocator, an aggregate, a string_literal, a character_literal, or an attribute_reference for an Access or Unchecked_Access attribute. Similarly, such an expression enclosed by parentheses is not allowed. A qualified_expression, see section Qualified Expressions, can be used instead of such a type_conversion.
  6. (22) The constraint of the target subtype has no effect for a type_conversion of an elementary type passed as an out parameter. Hence, it is recommended that the first subtype be specified as the target to minimize confusion (a similar recommendation applies to renaming and generic formal in out objects).

    Examples

  7. Examples of numeric type conversion: 
  8. Real(2*J)      --  value is converted to floating point
Integer(1.6)   --  value is 2
Integer(-0.4)  --  value is 0
  9. Example of conversion between derived types: 
  10. type A_Form is new B_Form;

  11. X : A_Form;
Y : B_Form;

  12. X := A_Form(Y);
Y := B_Form(X);  --  the reverse conversion
  13. Examples of conversions between array types: 
  14. type Sequence is array (Integer range <>) of Integer;
subtype Dozen is Sequence(1 .. 12);
Ledger : array(1 .. 100) of Integer;

  15. Sequence(Ledger)            --  bounds are those of Ledger
Sequence(Ledger(31 .. 42))  --  bounds are 31 and 42
Dozen(Ledger(31 .. 42))     --  bounds are those of Dozen

Qualified Expressions

  1. A qualified_expression is used to state explicitly the type, and to verify the subtype, of an operand that is either an expression or an aggregate.

    Syntax 

  2. qualified_expression ::=
   subtype_mark'(expression) | subtype_mark'aggregate

    Name Resolution Rules

  3. The operand (the expression or aggregate) shall resolve to be of the type determined by the subtype_mark, or a universal type that covers it.

    Dynamic Semantics

  4. The evaluation of a qualified_expression evaluates the operand (and if of a universal type, converts it to the type determined by the subtype_mark) and checks that its value belongs to the subtype denoted by the subtype_mark. The exception Constraint_Error is raised if this check fails. NOTES
  5. (23) When a given context does not uniquely identify an expected type, a qualified_expression can be used to do so. In particular, if an overloaded name or aggregate is passed to an overloaded subprogram, it might be necessary to qualify the operand to resolve its type.

    Examples

  6. Examples of disambiguating expressions using qualification: 
  7. type Mask is (Fix, Dec, Exp, Signif);
type Code is (Fix, Cla, Dec, Tnz, Sub);

  8. Print (Mask'(Dec));  --  Dec is of type Mask
Print (Code'(Dec));  --  Dec is of type Code

  9. for J in Code'(Fix) .. Code'(Dec) loop ...
-- qualification needed for either Fix or Dec

for J in Code range Fix .. Dec loop ...
-- qualification unnecessary

for J in Code'(Fix) .. Dec loop ...
-- qualification unnecessary for Dec

  10. Dozen'(1 | 3 | 5 | 7 => 2, others => 0) --  see section Type Conversions

Allocators

  1. The evaluation of an allocator creates an object and yields an access value that designates the object.

    Syntax 

  2. allocator ::=
   new subtype_indication | new qualified_expression

    Name Resolution Rules

  3. The expected type for an allocator shall be a single access-to-object type whose designated type covers the type determined by the subtype_mark of the subtype_indication or qualified_expression.

    Legality Rules

  4. An initialized allocator is an allocator with a qualified_expression. An uninitialized allocator is one with a subtype_indication. In the subtype_indication of an uninitialized allocator, a constraint is permitted only if the subtype_mark denotes an unconstrained composite subtype; if there is no constraint, then the subtype_mark shall denote a definite subtype.
  5. If the type of the allocator is an access-to-constant type, the allocator shall be an initialized allocator. If the designated type is limited, the allocator shall be an uninitialized allocator.

    Static Semantics

  6. If the designated type of the type of the allocator is elementary, then the subtype of the created object is the designated subtype. If the designated type is composite, then the created object is always constrained; if the designated subtype is constrained, then it provides the constraint of the created object; otherwise, the object is constrained by its initial value (even if the designated subtype is unconstrained with defaults).

    Dynamic Semantics

  7. For the evaluation of an allocator, the elaboration of the subtype_indication or the evaluation of the qualified_expression is performed first. For the evaluation of an initialized allocator, an object of the designated type is created and the value of the qualified_expression is converted to the designated subtype and assigned to the object.
  8. For the evaluation of an uninitialized allocator:
    1. If the designated type is elementary, an object of the designated subtype is created and any implicit initial value is assigned;
    2. If the designated type is composite, an object of the designated type is created with tag, if any, determined by the subtype_mark of the subtype_indication; any per-object constraints on subcomponents are elaborated and any implicit initial values for the subcomponents of the object are obtained as determined by the subtype_indication and assigned to the corresponding subcomponents. A check is made that the value of the object belongs to the designated subtype. Constraint_Error is raised if this check fails. This check and the initialization of the object are performed in an arbitrary order.
  1. If the created object contains any tasks, they are activated (see section Task Execution - Task Activation). Finally, an access value that designates the created object is returned. NOTES
  2. (24) Allocators cannot create objects of an abstract type (see section Abstract Types and Subprograms).
  3. (25) If any part of the created object is controlled, the initialization includes calls on corresponding Initialize or Adjust procedures (see section User-Defined Assignment and Finalization).
  4. (26) As explained in section Storage Management, the storage for an object allocated by an allocator comes from a storage pool (possibly user defined). The exception Storage_Error is raised by an allocator if there is not enough storage. Instances of Unchecked_Deallocation may be used to explicitly reclaim storage.
  5. (27) Implementations are permitted, but not required, to provide garbage collection, see section Pragma Controlled.

    Examples

  6. Examples of allocators: 
  7. new Cell'(0, null, null)
-- initialized explicitly, see section Incomplete Type Declarations

new Cell'(Value => 0, Succ => null, Pred => null)
-- initialized explicitly

new Cell
-- not initialized

  8. new Matrix(1 .. 10, 1 .. 20)
-- the bounds only are given

new Matrix'(1 .. 10 => (1 .. 20 => 0.0))
-- initialized explicitly

  9. new Buffer(100)
-- the discriminant only is given

new Buffer'(Size => 80, Pos => 0, Value => (1 .. 80 => 'A'))
-- initialized explicitly

  10. Expr_Ptr'(new Literal)
-- allocator for access-to-class-wide type, see section Type Extensions

Expr_Ptr'(new Literal'(Expression with 3.5))
-- initialized explicitly

Static Expressions and Static Subtypes

  1. Certain expressions of a scalar or string type are defined to be static. Similarly, certain discrete ranges are defined to be static, and certain scalar and string subtypes are defined to be static subtypes. Static means determinable at compile time, using the declared properties or values of the program entities.
  2. A static expression is a scalar or string expression that is one of the following:
    1. a numeric_literal;
    2. a string_literal of a static string subtype;
    3. a name that denotes the declaration of a named number or a static constant;
    4. a function_call whose function_name or function_prefix statically denotes a static function, and whose actual parameters, if any (whether given explicitly or by default), are all static expressions;
    5. an attribute_reference that denotes a scalar value, and whose prefix denotes a static scalar subtype;
    6. an attribute_reference whose prefix statically denotes a statically constrained array object or array subtype, and whose attribute_designator is First, Last, or Length, with an optional dimension;
    7. a type_conversion whose subtype_mark denotes a static scalar subtype, and whose operand is a static expression;
    8. a qualified_expression whose subtype_mark denotes a static (scalar or string) subtype, and whose operand is a static expression;
    9. a membership test whose simple_expression is a static expression, and whose range is a static range or whose subtype_mark denotes a static (scalar or string) subtype;
    10. a short-circuit control form both of whose relations are static expressions;
    11. a static expression enclosed in parentheses.
  1. A name statically denotes an entity if it denotes the entity and:
    1. It is a direct_name, expanded name, or character_literal, and it denotes a declaration other than a renaming_declaration; or
    2. It is an attribute_reference whose prefix statically denotes some entity; or
    3. It denotes a renaming_declaration with a name that statically denotes the renamed entity.
  1. A static function is one of the following:
    1. a predefined operator whose parameter and result types are all scalar types none of which are descendants of formal scalar types;
    2. a predefined concatenation operator whose result type is a string type;
    3. an enumeration literal;
    4. a language-defined attribute that is a function, if the prefix denotes a static scalar subtype, and if the parameter and result types are scalar.
  1. In any case, a generic formal subprogram is not a static function.
  2. A static constant is a constant view declared by a full constant declaration or an object_renaming_declaration with a static nominal subtype, having a value defined by a static scalar expression or by a static string expression whose value has a length not exceeding the maximum length of a string_literal in the implementation.
  3. A static range is a range whose bounds are static expressions, or a range_attribute_reference that is equivalent to such a range. A static discrete_range is one that is a static range or is a subtype_indication that defines a static scalar subtype. The base range of a scalar type is a static range, unless the type is a descendant of a formal scalar type.
  4. A static subtype is either a static scalar subtype or a static string subtype. A static scalar subtype is an unconstrained scalar subtype whose type is not a descendant of a formal scalar type, or a constrained scalar subtype formed by imposing a compatible static constraint on a static scalar subtype. A static string subtype is an unconstrained string subtype whose index subtype and component subtype are static (and whose type is not a descendant of a formal array type), or a constrained string subtype formed by imposing a compatible static constraint on a static string subtype. In any case, the subtype of a generic formal object of mode in out, and the result subtype of a generic formal function, are not static.
  5. The different kinds of static constraint are defined as follows:
    1. A null constraint is always static;
    2. A scalar constraint is static if it has no range_constraint, or one with a static range;
    3. An index constraint is static if each discrete_range is static, and each index subtype of the corresponding array type is static;
    4. A discriminant constraint is static if each expression of the constraint is static, and the subtype of each discriminant is static.
  1. A subtype is statically constrained if it is constrained, and its constraint is static. An object is statically constrained if its nominal subtype is statically constrained, or if it is a static string constant.

    Legality Rules

  2. A static expression is evaluated at compile time except when it is part of the right operand of a static short-circuit control form whose value is determined by its left operand. This evaluation is performed exactly, without performing Overflow_Checks. For a static expression that is evaluated:
    1. The expression is illegal if its evaluation fails a language-defined check other than Overflow_Check.
    2. If the expression is not part of a larger static expression, then its value shall be within the base range of its expected type. Otherwise, the value may be arbitrarily large or small.
    3. If the expression is of type universal_real and its expected type is a decimal fixed point type, then its value shall be a multiple of the small of the decimal type.
  1. The last two restrictions above do not apply if the expected type is a descendant of a formal scalar type (or a corresponding actual type in an instance).

    Implementation Requirements

  2. For a real static expression that is not part of a larger static expression, and whose expected type is not a descendant of a formal scalar type, the implementation shall round or truncate the value (according to the Machine_Rounds attribute of the expected type) to the nearest machine number of the expected type; if the value is exactly half-way between two machine numbers, any rounding shall be performed away from zero. If the expected type is a descendant of a formal scalar type, no special rounding or truncating is required -- normal accuracy rules apply, see section Numerics (normative). NOTES
  3. (28) An expression can be static even if it occurs in a context where staticness is not required.
  4. (29) A static (or run-time) type_conversion from a real type to an integer type performs rounding. If the operand value is exactly half-way between two integers, the rounding is performed away from zero.

    Examples

  5. Examples of static expressions: 
  6. 1 + 1       -- 2
abs(-10)*3  -- 30

  7. Kilo : constant := 1000;
Mega : constant := Kilo*Kilo;   -- 1_000_000
Long : constant := Float'Digits*2;

  8. Half_Pi    : constant := Pi/2;
--  see section Number Declarations.

Deg_To_Rad : constant := Half_Pi/90;

Rad_To_Deg : constant := 1.0/Deg_To_Rad;
-- equivalent to 1.0/((3.14159_26536/2)/90)

Statically Matching Constraints and Subtypes

Static Semantics

  1. A constraint statically matches another constraint if both are null constraints, both are static and have equal corresponding bounds or discriminant values, or both are nonstatic and result from the same elaboration of a constraint of a subtype_indication or the same evaluation of a range of a discrete_subtype_definition.
  2. A subtype statically matches another subtype of the same type if they have statically matching constraints. Two anonymous access subtypes statically match if their designated subtypes statically match.
  3. Two ranges of the same type statically match if both result from the same evaluation of a range, or if both are static and have equal corresponding bounds.
  4. A constraint is statically compatible with a scalar subtype if it statically matches the constraint of the subtype, or if both are static and the constraint is compatible with the subtype. A constraint is statically compatible with an access or composite subtype if it statically matches the constraint of the subtype, or if the subtype is unconstrained. One subtype is statically compatible with a second subtype if the constraint of the first is statically compatible with the second subtype.

Statements

  1. A statement defines an action to be performed upon its execution.
  2. This section describes the general rules applicable to all statements. Some statements are discussed in later sections: Procedure_call_statements and return_statements are described in section Subprograms. Entry_call_statements, requeue_statements, delay_statements, accept_statements, select_statements, and abort_statements are described in section Tasks and Synchronization. Raise_statements are described in section Exceptions, and code_statements in section Representation Issues. The remaining forms of statements are presented in this section.

Simple and Compound Statements - Sequences of Statements

  1. A statement is either simple or compound. A simple_statement encloses no other statement. A compound_statement can enclose simple_statements and other compound_statements.

    Syntax 

  2. sequence_of_statements ::= statement {statement}

  3. statement ::=
   {label} simple_statement | {label} compound_statement

  4. simple_statement ::= null_statement
   | assignment_statement   | exit_statement
   | goto_statement         | procedure_call_statement
   | return_statement       | entry_call_statement
   | requeue_statement      | delay_statement
   | abort_statement        | raise_statement
   | code_statement

  5. compound_statement ::=
     if_statement           | case_statement
   | loop_statement         | block_statement
   | accept_statement       | select_statement

  6. null_statement ::= null;

  7. label ::= <<label_statement_identifier>>

  8. statement_identifier ::= direct_name
    1. The direct_name of a statement_identifier shall be an identifier (not an operator_symbol).

Name Resolution Rules

  1. The direct_name of a statement_identifier shall resolve to denote its corresponding implicit declaration (see below).

    Legality Rules

  2. Distinct identifiers shall be used for all statement_identifiers that appear in the same body, including inner block_statements but excluding inner program units.

    Static Semantics

  3. For each statement_identifier, there is an implicit declaration (with the specified identifier) at the end of the declarative_part of the innermost block_statement or body that encloses the statement_identifier. The implicit declarations occur in the same order as the statement_identifiers occur in the source text. If a usage name denotes such an implicit declaration, the entity it denotes is the label, loop_statement, or block_statement with the given statement_identifier.

    Dynamic Semantics

  4. The execution of a null_statement has no effect.
  5. A transfer of control is the run-time action of an exit_statement, return_statement, goto_statement, or requeue_statement, selection of a terminate_alternative, raising of an exception, or an abort, which causes the next action performed to be one other than what would normally be expected from the other rules of the language. As explained in section Completion and Finalization, a transfer of control can cause the execution of constructs to be completed and then left, which may trigger finalization.
  6. The execution of a sequence_of_statements consists of the execution of the individual statements in succession until the sequence_ is completed. NOTES
  7. (1) A statement_identifier that appears immediately within the declarative region of a named loop_statement or an accept_statement is nevertheless implicitly declared immediately within the declarative region of the innermost enclosing body or block_statement; in other words, the expanded name for a named statement is not affected by whether the statement occurs inside or outside a named loop or an accept_statement -- only nesting within block_statements is relevant to the form of its expanded name.

    Examples

  8. Examples of labeled statements: 
  9. <<Here>> <<Ici>> <<Aqui>> <<Hier>> null;

  10. <<After>> X := 1;

Assignment Statements

  1. An assignment_statement replaces the current value of a variable with the result of evaluating an expression.

    Syntax 

  2. assignment_statement ::= variable_name := expression;
  3. The execution of an assignment_statement includes the evaluation of the expression and the assignment of the value of the expression into the target. An assignment operation (as opposed to an assignment_statement) is performed in other contexts as well, including object initialization and by-copy parameter passing. The target of an assignment operation is the view of the object to which a value is being assigned; the target of an assignment_statement is the variable denoted by the variable_name.

    Name Resolution Rules

  4. The variable_name of an assignment_statement is expected to be of any nonlimited type. The expected type for the expression is the type of the target.

    Legality Rules

  5. The target denoted by the variable_name shall be a variable.
  6. If the target is of a tagged class-wide type T'Class, then the expression shall either be dynamically tagged, or of type T and tag-indeterminate, see section Dispatching Operations of Tagged Types.

    Dynamic Semantics

  7. For the execution of an assignment_statement, the variable_name and the expression are first evaluated in an arbitrary order.
  8. When the type of the target is class-wide:
    1. If the expression is tag-indeterminate, see section Dispatching Operations of Tagged Types, then the controlling tag value for the expression is the tag of the target;
    2. Otherwise (the expression is dynamically tagged), a check is made that the tag of the value of the expression is the same as that of the target; if this check fails, Constraint_Error is raised.
  1. The value of the expression is converted to the subtype of the target. The conversion might raise an exception, see section Type Conversions.
  2. In cases involving controlled types, the target is finalized, and an anonymous object might be used as an intermediate in the assignment, as described in section Completion and Finalization. In any case, the converted value of the expression is then assigned to the target, which consists of the following two steps:
    1. The value of the target becomes the converted value.
    2. If any part of the target is controlled, its value is adjusted as explained in section User-Defined Assignment and Finalization.
    NOTES
  1. (2) The tag of an object never changes; in particular, an assignment_statement does not change the tag of the target.
  2. (3) The values of the discriminants of an object designated by an access value cannot be changed (not even by assigning a complete value to the object itself) since such objects are always constrained; however, subcomponents of such objects may be unconstrained.

    Examples

  3. Examples of assignment statements: 
  4. Value := Max_Value - 1;
Shade := Blue;

  5. Next_Frame(F)(M, N) := 2.5;  --  see section Indexed Components
U := Dot_Product(V, W);      --  see section Subprogram Bodies

  6. Writer := (Status => Open, Unit => Printer, Line_Count => 60);
--  see section Variant Parts and Discrete Choices

Next_Car.all := (72074, null);
--  see section Incomplete Type Declarations
  7. Examples involving scalar subtype conversions: 
  8. I, J : Integer range 1 .. 10 := 5;
K    : Integer range 1 .. 20 := 15;
 ...

  9. I := J;  --  identical ranges
K := J;  --  compatible ranges
J := K;  --  will raise Constraint_Error if K > 10
  10. Examples involving array subtype conversions: 
  11. A : String(1 .. 31);
B : String(3 .. 33);
 ...

  12. A := B;  --  same number of components

  13. A(1 .. 9)  := "tar sauce";
A(4 .. 12) := A(1 .. 9);  --  A(1 .. 12) = "tartar sauce"
    NOTES
  14. (4) Notes on the examples: Assignment_statements are allowed even in the case of overlapping slices of the same array, because the variable_name and expression are both evaluated before copying the value into the variable. In the above example, an implementation yielding A(1 .. 12) = "tartartartar" would be incorrect.

If Statements

  1. An if_statement selects for execution at most one of the enclosed sequences_of_statements, depending on the (truth) value of one or more corresponding conditions.

    Syntax 

  2. if_statement ::=
    if condition then
       sequence_of_statements
    {elsif condition then
       sequence_of_statements}
    [else
       sequence_of_statements]
    end if;

  3. condition ::= boolean_expression

    Name Resolution Rules

  4. A condition is expected to be of any boolean type.

    Dynamic Semantics

  5. For the execution of an if_statement, the condition specified after if, and any conditions specified after elsif, are evaluated in succession (treating a final else as elsif True then), until one evaluates to True or all conditions are evaluated and yield False. If a condition evaluates to True, then the corresponding sequence_of_statements is executed; otherwise none of them is executed.

    Examples

  6. Examples of if statements: 
  7. if Month = December and Day = 31 then
   Month := January;
   Day   := 1;
   Year  := Year + 1;
end if;

  8. if Line_Too_Short then
   raise Layout_Error;
elsif Line_Full then
   New_Line;
   Put(Item);
else
   Put(Item);
end if;

  9. if My_Car.Owner.Vehicle /= My_Car then  --  see section Incomplete Type Declarations
   Report ("Incorrect data");
end if;

Case Statements

  1. A case_statement selects for execution one of a number of alternative sequences_of_statements; the chosen alternative is defined by the value of an expression.

    Syntax 

  2. case_statement ::=
   case expression is
      case_statement_alternative
      {case_statement_alternative}
   end case;

  3. case_statement_alternative ::=
   when discrete_choice_list =>
      sequence_of_statements

    Name Resolution Rules

  4. The expression is expected to be of any discrete type. The expected type for each discrete_choice is the type of the expression.

    Legality Rules

  5. The expressions and discrete_ranges given as discrete_choices of a case_statement shall be static. A discrete_choice others, if present, shall appear alone and in the last discrete_choice_list.
  6. The possible values of the expression shall be covered as follows:
    1. If the expression is a name (including a type_conversion or a function_call) having a static and constrained nominal subtype, or is a qualified_expression whose subtype_mark denotes a static and constrained scalar subtype, then each non-others discrete_choice shall cover only values in that subtype, and each value of that subtype shall be covered by some discrete_choice (either explicitly or by others).
    2. If the type of the expression is root_integer, universal_integer, or a descendant of a formal scalar type, then the case_statement shall have an others discrete_choice.
    3. Otherwise, each value of the base range of the type of the expression shall be covered (either explicitly or by others).
  1. Two distinct discrete_choices of a case_statement shall not cover the same value.

    Dynamic Semantics

  2. For the execution of a case_statement the expression is first evaluated.
  3. If the value of the expression is covered by the discrete_choice_list of some case_statement_alternative, then the sequence_of_statements of the _alternative is executed.
  4. Otherwise (the value is not covered by any discrete_choice_list, perhaps due to being outside the base range), Constraint_Error is raised. NOTES
  5. (5) The execution of a case_statement chooses one and only one alternative. Qualification of the expression of a case_statement by a static subtype can often be used to limit the number of choices that need be given explicitly.

    Examples

  6. Examples of case statements: 
  7. case Sensor is
   when Elevation  => Record_Elevation(Sensor_Value);
   when Azimuth    => Record_Azimuth  (Sensor_Value);
   when Distance   => Record_Distance (Sensor_Value);
   when others     => null;
end case;

  8. case Today is
   when Mon        => Compute_Initial_Balance;
   when Fri        => Compute_Closing_Balance;
   when Tue .. Thu => Generate_Report(Today);
   when Sat .. Sun => null;
end case;

  9. case Bin_Number(Count) is
   when 1      => Update_Bin(1);
   when 2      => Update_Bin(2);
   when 3 | 4  =>
      Empty_Bin(1);
      Empty_Bin(2);
   when others => raise Error;
end case;

Loop Statements

  1. A loop_statement includes a sequence_of_statements that is to be executed repeatedly, zero or more times.

    Syntax 

  2. loop_statement ::=
   [loop_statement_identifier:]
      [iteration_scheme] loop
         sequence_of_statements
      end loop [loop_identifier];

  3. iteration_scheme ::= while condition
   | for loop_parameter_specification

  4. loop_parameter_specification ::=
   defining_identifier in [reverse] discrete_subtype_definition
    1. If a loop_statement has a loop_statement_identifier, then the identifier shall be repeated after the end loop; otherwise, there shall not be an identifier after the end loop.

Static Semantics

  1. A loop_parameter_specification declares a loop parameter, which is an object whose subtype is that defined by the discrete_subtype_definition.

    Dynamic Semantics

  2. For the execution of a loop_statement, the sequence_of_statements is executed repeatedly, zero or more times, until the loop_statement is complete. The loop_statement is complete when a transfer of control occurs that transfers control out of the loop, or, in the case of an iteration_scheme, as specified below.
  3. For the execution of a loop_statement with a while iteration_scheme, the condition is evaluated before each execution of the sequence_of_statements; if the value of the condition is True, the sequence_of_statements is executed; if False, the execution of the loop_statement is complete.
  4. For the execution of a loop_statement with a for iteration_scheme, the loop_parameter_specification is first elaborated. This elaboration creates the loop parameter and elaborates the discrete_subtype_definition. If the discrete_subtype_definition defines a subtype with a null range, the execution of the loop_statement is complete. Otherwise, the sequence_of_statements is executed once for each value of the discrete subtype defined by the discrete_subtype_definition (or until the loop is left as a consequence of a transfer of control). Prior to each such iteration, the corresponding value of the discrete subtype is assigned to the loop parameter. These values are assigned in increasing order unless the reserved word reverse is present, in which case the values are assigned in decreasing order. NOTES
  5. (6) A loop parameter is a constant; it cannot be updated within the sequence_of_statements of the loop, see section Objects and Named Numbers.
  6. (7) An object_declaration should not be given for a loop parameter, since the loop parameter is automatically declared by the loop_parameter_specification. The scope of a loop parameter extends from the loop_parameter_specification to the end of the loop_statement, and the visibility rules are such that a loop parameter is only visible within the sequence_of_statements of the loop.
  7. (8) The discrete_subtype_definition of a for loop is elaborated just once. Use of the reserved word reverse does not alter the discrete subtype defined, so that the following iteration_schemes are not equivalent; the first has a null range. 
  8. for J in reverse 1 ..  0
for J in 0 .. 1

    Examples

  9. Example of a loop statement without an iteration scheme: 
  10. loop
   Get(Current_Character);
   exit when Current_Character = '*';
end loop;
  11. Example of a loop statement with a while iteration scheme: 
  12. while Bid(N).Price < Cut_Off.Price loop
   Record_Bid(Bid(N).Price);
   N := N + 1;
end loop;
  13. Example of a loop statement with a for iteration scheme: 
  14. for J in Buffer'Range loop  --  works even with a null range
   if Buffer(J) /= Space then
      Put(Buffer(J));
   end if;
end loop;
  15. Example of a loop statement with a name: 
  16. Summation:
   while Next /= Head loop  --  see section Incomplete Type Declarations
      Sum  := Sum + Next.Value;
      Next := Next.Succ;
   end loop Summation;

Block Statements

  1. A block_statement encloses a handled_sequence_of_statements optionally preceded by a declarative_part.

    Syntax 

  2. block_statement ::=
   [block_statement_identifier:]
      [declare
         declarative_part]
      begin
         handled_sequence_of_statements
      end [block_identifier];
    1. If a block_statement has a block_statement_identifier, then the identifier shall be repeated after the end; otherwise, there shall not be an identifier after the end.

Static Semantics

  1. A block_statement that has no explicit declarative_part has an implicit empty declarative_part.

    Dynamic Semantics

  2. The execution of a block_statement consists of the elaboration of its declarative_part followed by the execution of its handled_sequence_of_statements.

    Examples

  3. Example of a block statement with a local variable: 
  4. Swap:
   declare
      Temp : Integer;
   begin
      Temp := V; V := U; U := Temp;
   end Swap;

Exit Statements

  1. An exit_statement is used to complete the execution of an enclosing loop_statement; the completion is conditional if the exit_statement includes a condition.

    Syntax 

  2. exit_statement ::= exit [loop_name] [when condition];

    Name Resolution Rules

  3. The loop_name, if any, in an exit_statement shall resolve to denote a loop_statement.

    Legality Rules

  4. Each exit_statement applies to a loop_statement; this is the loop_statement being exited. An exit_statement with a name is only allowed within the loop_statement denoted by the name, and applies to that loop_statement. An exit_statement without a name is only allowed within a loop_statement, and applies to the innermost enclosing one. An exit_statement that applies to a given loop_statement shall not appear within a body or accept_statement, if this construct is itself enclosed by the given loop_statement.

    Dynamic Semantics

  5. For the execution of an exit_statement, the condition, if present, is first evaluated. If the value of the condition is True, or if there is no condition, a transfer of control is done to complete the loop_statement. If the value of the condition is False, no transfer of control takes place. NOTES
  6. (9) Several nested loops can be exited by an exit_statement that names the outer loop.

    Examples

  7. Examples of loops with exit statements: 
  8. for N in 1 .. Max_Num_Items loop
   Get_New_Item(New_Item);
   Merge_Item(New_Item, Storage_File);
   exit when New_Item = Terminal_Item;
end loop;

  9. Main_Cycle:
   loop
      --  initial statements
      exit Main_Cycle when Found;
      --  final statements
   end loop Main_Cycle;

Goto Statements

  1. A goto_statement specifies an explicit transfer of control from this statement to a target statement with a given label.

    Syntax 

  2. goto_statement ::= goto label_name;

    Name Resolution Rules

  3. The label_name shall resolve to denote a label; the statement with that label is the target statement.

    Legality Rules

  4. The innermost sequence_of_statements that encloses the target statement shall also enclose the goto_statement. Furthermore, if a goto_statement is enclosed by an accept_statement or a body, then the target statement shall not be outside this enclosing construct.

    Dynamic Semantics

  5. The execution of a goto_statement transfers control to the target statement, completing the execution of any compound_statement that encloses the goto_statement but does not enclose the target. NOTES
  6. (10) The above rules allow transfer of control to a statement of an enclosing sequence_of_statements but not the reverse. Similarly, they prohibit transfers of control such as between alternatives of a case_statement, if_statement, or select_statement; between exception_handlers; or from an exception_handler of a handled_sequence_of_statements back to its sequence_of_statements.

    Examples

  7. Example of a loop containing a goto statement: 
  8. <<Sort>>
for I in 1 .. N-1 loop
   if A(I) > A(I+1) then
      Exchange(A(I), A(I+1));
      goto Sort;
   end if;
end loop;

Subprograms

  1. A subprogram is a program unit or intrinsic operation whose execution is invoked by a subprogram call. There are two forms of subprogram: procedures and functions. A procedure call is a statement; a function call is an expression and returns a value. The definition of a subprogram can be given in two parts: a subprogram declaration defining its interface, and a subprogram_body defining its execution. Operators and enumeration literals are functions.
  2. A callable entity is a subprogram or entry, see section Tasks and Synchronization. A callable entity is invoked by a call; that is, a subprogram call or entry call. A callable construct is a construct that defines the action of a call upon a callable entity: a subprogram_body, entry_body, or accept_statement.

Subprogram Declarations

  1. A subprogram_declaration declares a procedure or function.

    Syntax 

  2. subprogram_declaration ::= subprogram_specification;

  3. abstract_subprogram_declaration ::=
   subprogram_specification is abstract;

  4. subprogram_specification ::=
     procedure defining_program_unit_name parameter_profile
   | function defining_designator parameter_and_result_profile

  5. designator ::= [parent_unit_name . ]identifier | operator_symbol

  6. defining_designator ::=
   defining_program_unit_name | defining_operator_symbol

  7. defining_program_unit_name ::=
   [parent_unit_name . ]defining_identifier
    1. The optional parent_unit_name is only allowed for library units (see section Compilation Units - Library Units). 
  1. operator_symbol ::= string_literal
    1. The sequence of characters in an operator_symbol shall correspond to an operator belonging to one of the six classes of operators defined in clause section Operators and Expression Evaluation (spaces are not allowed and the case of letters is not significant). 
  1. defining_operator_symbol ::= operator_symbol

  2. parameter_profile ::= [formal_part]

  3. parameter_and_result_profile ::= [formal_part] return subtype_mark

  4. formal_part ::=
   (parameter_specification {; parameter_specification{)

  5. parameter_specification ::=
    defining_identifier_list : mode subtype_mark
      [:= default_expression]
  | defining_identifier_list : access_definition
      [:= default_expression]

  6. mode ::= [in] | in out | out

    Name Resolution Rules

  7. A formal parameter is an object directly visible within a subprogram_body that represents the actual parameter passed to the subprogram in a call; it is declared by a parameter_specification. For a formal parameter, the expected type for its default_expression, if any, is that of the formal parameter.

    Legality Rules

  8. The parameter mode of a formal parameter conveys the direction of information transfer with the actual parameter: in, in out, or out. Mode in is the default, and is the mode of a parameter defined by an access_definition. The formal parameters of a function, if any, shall have the mode in.
  9. A default_expression is only allowed in a parameter_specification for a formal parameter of mode in.
  10. A subprogram_declaration or a generic_subprogram_declaration requires a completion: a body, a renaming_declaration, see section Renaming Declarations, or a pragma Import, see section Interfacing Pragmas. A completion is not allowed for an abstract_subprogram_declaration.
  11. A name that denotes a formal parameter is not allowed within the formal_part in which it is declared, nor within the formal_part of a corresponding body or accept_statement.

    Static Semantics

  12. The profile of (a view of) a callable entity is either a parameter_profile or parameter_and_result_profile; it embodies information about the interface to that entity -- for example, the profile includes information about parameters passed to the callable entity. All callable entities have a profile -- enumeration literals, other subprograms, and entries. An access-to-subprogram type has a designated profile. Associated with a profile is a calling convention. A subprogram_declaration declares a procedure or a function, as indicated by the initial reserved word, with name and profile as given by its specification.
  13. The nominal subtype of a formal parameter is the subtype denoted by the subtype_mark, or defined by the access_definition, in the parameter_specification.
  14. An access parameter is a formal in parameter specified by an access_definition. An access parameter is of an anonymous general access-to-variable type, see section Access Types. Access parameters allow dispatching calls to be controlled by access values.
  15. The subtypes of a profile are:
    1. For any non-access parameters, the nominal subtype of the parameter.
    2. For any access parameters, the designated subtype of the parameter type.
    3. For any result, the result subtype.
  1. The types of a profile are the types of those subtypes.
  2. A subprogram declared by an abstract_subprogram_declaration is abstract; a subprogram declared by a subprogram_declaration is not (see section Abstract Types and Subprograms).

    Dynamic Semantics

  3. The elaboration of a subprogram_declaration or an abstract_subprogram_declaration has no effect. NOTES
  4. (1) A parameter_specification with several identifiers is equivalent to a sequence of single parameter_specifications, as explained in 3.3.
  5. (2) Abstract subprograms do not have bodies, and cannot be used in a nondispatching call, see section Abstract Types and Subprograms.
  6. (3) The evaluation of default_expressions is caused by certain calls, as described in section Parameter Associations. They are not evaluated during the elaboration of the subprogram declaration.
  7. (4) Subprograms can be called recursively and can be called concurrently from multiple tasks.

    Examples

  8. Examples of subprogram declarations: 
  9. procedure Traverse_Tree;
procedure Increment(X : in out Integer);
procedure Right_Indent(Margin : out Line_Size); --  see section Integer Types
procedure Switch(From, To : in out Link);       --  see section Incomplete Type Declarations

  10. function Random return Probability;             --  see section Floating Point Types

  11. function Min_Cell(X : Link) return Cell;        --  see section Incomplete Type Declarations
function Next_Frame(K : Positive) return Frame; --  see section Access Types
function Dot_Product(Left, Right : Vector) return Real;
--  see section Array Types

  12. function "*"(Left, Right : Matrix) return Matrix;
--  see section Array Types
  13. Examples of in parameters with default expressions: 
  14. procedure Print_Header
  (Pages  : in Natural;
   Header : in Line    :=  (1 .. Line'Last => ' '); --  see section Array Types
   Center : in Boolean := True);

Formal Parameter Modes

  1. A parameter_specification declares a formal parameter of mode in, in out, or out.

    Static Semantics

  2. A parameter is passed either by copy or by reference. When a parameter is passed by copy, the formal parameter denotes a separate object from the actual parameter, and any information transfer between the two occurs only before and after executing the subprogram. When a parameter is passed by reference, the formal parameter denotes (a view of) the object denoted by the actual parameter; reads and updates of the formal parameter directly reference the actual parameter object.
  3. A type is a by-copy type if it is an elementary type, or if it is a descendant of a private type whose full type is a by-copy type. A parameter of a by-copy type is passed by copy.
  4. A type is a by-reference type if it is a descendant of one of the following:
    1. a tagged type;
    2. a task or protected type;
    3. a nonprivate type with the reserved word limited in its declaration;
    4. a composite type with a subcomponent of a by-reference type;
    5. a private type whose full type is a by-reference type.
  1. A parameter of a by-reference type is passed by reference. Each value of a by-reference type has an associated object. For a parenthesized expression, qualified_expression, or type_conversion, this object is the one associated with the operand.
  2. For parameters of other types, it is unspecified whether the parameter is passed by copy or by reference.

    Bounded (Run-Time) Errors

  3. If one name denotes a part of a formal parameter, and a second name denotes a part of a distinct formal parameter or an object that is not part of a formal parameter, then the two names are considered distinct access paths. If an object is of a type for which the parameter passing mechanism is not specified, then it is a bounded error to assign to the object via one access path, and then read the value of the object via a distinct access path, unless the first access path denotes a part of a formal parameter that no longer exists at the point of the second access (due to leaving the corresponding callable construct). The possible consequences are that Program_Error is raised, or the newly assigned value is read, or some old value of the object is read. NOTES
  4. (5) A formal parameter of mode in is a constant view, see section Objects and Named Numbers, it cannot be updated within the subprogram_body.

Subprogram Bodies

  1. A subprogram_body specifies the execution of a subprogram.

    Syntax 

  2. subprogram_body ::=
   subprogram_specification is
      declarative_part
   begin
      handled_sequence_of_statements
   end [designator];
    1. If a designator appears at the end of a subprogram_body, it shall repeat the defining_designator of the subprogram_specification.

Legality Rules

  1. In contrast to other bodies, a subprogram_body need not be the completion of a previous declaration, in which case the body declares the subprogram. If the body is a completion, it shall be the completion of a subprogram_declaration or generic_subprogram_declaration. The profile of a subprogram_body that completes a declaration shall conform fully to that of the declaration.

    Static Semantics

  2. A subprogram_body is considered a declaration. It can either complete a previous declaration, or itself be the initial declaration of the subprogram.

    Dynamic Semantics

  3. The elaboration of a non-generic subprogram_body has no other effect than to establish that the subprogram can from then on be called without failing the Elaboration_Check.
  4. The execution of a subprogram_body is invoked by a subprogram call. For this execution the declarative_part is elaborated, and the handled_sequence_of_statements is then executed.

    Examples

  5. Example of procedure body: 
  6. procedure Push(E : in Element_Type; S : in out Stack) is
begin
   if S.Index = S.Size then
      raise Stack_Overflow;
   else
      S.Index := S.Index + 1;
      S.Space(S.Index) := E;
   end if;
end Push;
  7. Example of a function body: 
  8. function Dot_Product(Left, Right : Vector) return Real is
   Sum : Real := 0.0;
begin
   Check(Left'First = Right'First and Left'Last = Right'Last);
   for J in Left'Range loop
      Sum := Sum + Left(J)*Right(J);
   end loop;
   return Sum;
end Dot_Product;

Conformance Rules

  1. When subprogram profiles are given in more than one place, they are required to conform in one of four ways: type conformance, mode conformance, subtype conformance, or full conformance.

    Static Semantics

  2. As explained in section Interfacing Pragmas, a convention can be specified for an entity. For a callable entity or access-to-subprogram type, the convention is called the calling convention. The following conventions are defined by the language:
    1. The default calling convention for any subprogram not listed below is Ada. A pragma Convention, Import, or Export may be used to override the default calling convention, see section Interfacing Pragmas.
    2. The Intrinsic calling convention represents subprograms that are "built in" to the compiler. The default calling convention is Intrinsic for the following:
        1. an enumeration literal;
        2. a "/=" operator declared implicitly due to the declaration of "=" (see section Overloading of Operators);
        3. any other implicitly declared subprogram unless it is a dispatching operation of a tagged type;
        4. an inherited subprogram of a generic formal tagged type with unknown discriminants;
        5. an attribute that is a subprogram;
        6. a subprogram declared immediately within a protected_body.
      1. The Access attribute is not allowed for Intrinsic subprograms.
    1. The default calling convention is protected for a protected subprogram, and for an access-to-subprogram type with the reserved word protected in its definition.
    2. The default calling convention is entry for an entry.
  1. Of these four conventions, only Ada and Intrinsic are allowed as a convention_identifier in a pragma Convention, Import, or Export.
  2. Two profiles are type conformant if they have the same number of parameters, and both have a result if either does, and corresponding parameter and result types are the same, or, for access parameters, corresponding designated types are the same.
  3. Two profiles are mode conformant if they are type-conformant, and corresponding parameters have identical modes, and, for access parameters, the designated subtypes statically match.
  4. Two profiles are subtype conformant if they are mode-conformant, corresponding subtypes of the profile statically match, and the associated calling conventions are the same. The profile of a generic formal subprogram is not subtype-conformant with any other profile.
  5. Two profiles are fully conformant if they are subtype-conformant, and corresponding parameters have the same names and have default_expressions that are fully conformant with one another.
  6. Two expressions are fully conformant if, after replacing each use of an operator with the equivalent function_call:
    1. each constituent construct of one corresponds to an instance of the same syntactic category in the other, except that an expanded name may correspond to a direct_name (or character_literal) or to a different expanded name in the other; and
    2. each direct_name, character_literal, and selector_name that is not part of the prefix of an expanded name in one denotes the same declaration as the corresponding direct_name, character_literal, or selector_name in the other; and
    3. each primary that is a literal in one has the same value as the corresponding literal in the other.
  1. Two known_discriminant_parts are fully conformant if they have the same number of discriminants, and discriminants in the same positions have the same names, statically matching subtypes, and default_expressions that are fully conformant with one another.
  2. Two discrete_subtype_definitions are fully conformant if they are both subtype_indications or are both ranges, the subtype_marks (if any) denote the same subtype, and the corresponding simple_expressions of the ranges (if any) fully conform.

    Implementation Permissions

  3. An implementation may declare an operator declared in a language-defined library unit to be intrinsic.

Inline Expansion of Subprograms

  1. Subprograms may be expanded in line at the call site.

    Syntax

  2. The form of a pragma Inline, which is a program unit pragma (see section Pragmas and Program Units) is as follows: 
  3. pragma Inline(name {, name{);

    Legality Rules

  4. The pragma shall apply to one or more callable entities or generic subprograms.

    Static Semantics

  5. If a pragma Inline applies to a callable entity, this indicates that inline expansion is desired for all calls to that entity. If a pragma Inline applies to a generic subprogram, this indicates that inline expansion is desired for all calls to all instances of that generic subprogram.

    Implementation Permissions

  6. For each call, an implementation is free to follow or to ignore the recommendation expressed by the pragma. NOTES
  7. (6) The name in a pragma Inline can denote more than one entity in the case of overloading. Such a pragma applies to all of the denoted entities.

Subprogram Calls

  1. A subprogram call is either a procedure_call_statement or a function_call; it invokes the execution of the subprogram_body. The call specifies the association of the actual parameters, if any, with formal parameters of the subprogram.

    Syntax 

  2. procedure_call_statement ::=
     procedure_name;
   | procedure_prefix actual_parameter_part;

  3. function_call ::=
     function_name
   | function_prefix actual_parameter_part

  4. actual_parameter_part ::=
   (parameter_association {, parameter_association{)

  5. parameter_association ::=
   [formal_parameter_selector_name =>] explicit_actual_parameter

  6. explicit_actual_parameter ::= expression | variable_name
    1. A parameter_association is named or positional according to whether or not the formal_parameter_selector_name is specified. Any positional associations shall precede any named associations. Named associations are not allowed if the prefix in a subprogram call is an attribute_reference.

Name Resolution Rules

  1. The name or prefix given in a procedure_call_statement shall resolve to denote a callable entity that is a procedure, or an entry renamed as (viewed as) a procedure. The name or prefix given in a function_call shall resolve to denote a callable entity that is a function. When there is an actual_parameter_part, the prefix can be an implicit_dereference of an access-to-subprogram value.
  2. A subprogram call shall contain at most one association for each formal parameter. Each formal parameter without an association shall have a default_expression (in the profile of the view denoted by the name or prefix). This rule is an overloading rule, see section The Context of Overload Resolution.

    Dynamic Semantics

  3. For the execution of a subprogram call, the name or prefix of the call is evaluated, and each parameter_association is evaluated, see section Parameter Associations. If a default_expression is used, an implicit parameter_association is assumed for this rule. These evaluations are done in an arbitrary order. The subprogram_body is then executed. Finally, if the subprogram completes normally, then after it is left, any necessary assigning back of formal to actual parameters occurs, see section Parameter Associations.
  4. The exception Program_Error is raised at the point of a function_call if the function completes normally without executing a return_statement.
  5. A function_call denotes a constant, as defined in section Return Statements, the nominal subtype of the constant is given by the result subtype of the function.

    Examples

  6. Examples of procedure calls: 
  7. Traverse_Tree;                   --  see section Subprogram Declarations
Print_Header(128, Title, True);  --  see section Subprogram Declarations

  8. Switch(From => X, To => Next);
--  see section Subprogram Declarations.

Print_Header(128, Header => Title, Center => True);
--  see section Subprogram Declarations.

Print_Header(Header => Title, Center => True, Pages => 128);
--  see section Subprogram Declarations.
  9. Examples of function calls: 
  10. Dot_Product(U, V)   --  see section Subprogram Declarations, and section Subprogram Bodies.
Clock               --  see section Delay Statements, Duration, and Time.
F.all
--  presuming F is of an access-to-subprogram type
--  see section Access Types.
  11. Examples of procedures with default expressions: 
  12. procedure Activate(Process : in Process_Name;
                   After   : in Process_Name := No_Process;
                   Wait    : in Duration := 0.0;
                   Prior   : in Boolean := False);

  13. procedure Pair(Left, Right : in Person_Name := new Person);
--  see section Incomplete Type Declarations.
  14. Examples of their calls: 
  15. Activate(X);
Activate(X, After => Y);
Activate(X, Wait => 60.0, Prior => True);
Activate(X, Y, 10.0, False);

  16. Pair;
Pair(Left => new Person, Right => new Person);
    NOTES
  17. (7) If a default_expression is used for two or more parameters in a multiple parameter_specification, the default_expression is evaluated once for each omitted parameter. Hence in the above examples, the two calls of Pair are equivalent.

    Examples

  18. Examples of overloaded subprograms: 
  19. procedure Put(X : in Integer);
procedure Put(X : in String);

  20. procedure Set(Tint   : in Color);
procedure Set(Signal : in Light);
  21. Examples of their calls: 
  22. Put(28);
Put("no possible ambiguity here");

  23. Set(Tint   => Red);
Set(Signal => Red);
Set(Color'(Red));

  24. --  Set(Red) would be ambiguous since Red may
--  denote a value either of type Color or of type Light

Parameter Associations

  1. A parameter association defines the association between an actual parameter and a formal parameter.

    Name Resolution Rules

  2. The formal_parameter_selector_name of a parameter_association shall resolve to denote a parameter_specification of the view being called.
  3. The actual parameter is either the explicit_actual_parameter given in a parameter_association for a given formal parameter, or the corresponding default_expression if no parameter_association is given for the formal parameter. The expected type for an actual parameter is the type of the corresponding formal parameter.
  4. If the mode is in, the actual is interpreted as an expression; otherwise, the actual is interpreted only as a name, if possible.

    Legality Rules

  5. If the mode is in out or out, the actual shall be a name that denotes a variable.
  6. The type of the actual parameter associated with an access parameter shall be convertible, see section Type Conversions, to its anonymous access type.

    Dynamic Semantics

  7. For the evaluation of a parameter_association:
    1. The actual parameter is first evaluated.
    2. For an access parameter, the access_definition is elaborated, which creates the anonymous access type.
    3. For a parameter (of any mode) that is passed by reference, see section Formal Parameter Modes, a view conversion of the actual parameter to the nominal subtype of the formal parameter is evaluated, and the formal parameter denotes that conversion.
    4. For an in or in out parameter that is passed by copy, see section Formal Parameter Modes, the formal parameter object is created, and the value of the actual parameter is converted to the nominal subtype of the formal parameter and assigned to the formal.
    5. For an out parameter that is passed by copy, the formal parameter object is created, and:
      1. For an access type, the formal parameter is initialized from the value of the actual, without a constraint check;
      2. For a composite type with discriminants or that has implicit initial values for any subcomponents, see section Object Declarations, the behavior is as for an in out parameter passed by copy.
      3. For any other type, the formal parameter is uninitialized. If composite, a view conversion of the actual parameter to the nominal subtype of the formal is evaluated (which might raise Constraint_Error), and the actual subtype of the formal is that of the view conversion. If elementary, the actual subtype of the formal is given by its nominal subtype.
  1. A formal parameter of mode in out or out with discriminants is constrained if either its nominal subtype or the actual parameter is constrained.
  2. After normal completion and leaving of a subprogram, for each in out or out parameter that is passed by copy, the value of the formal parameter is converted to the subtype of the variable given as the actual parameter and assigned to it. These conversions and assignments occur in an arbitrary order.

Return Statements

  1. A return_statement is used to complete the execution of the innermost enclosing subprogram_body, entry_body, or accept_statement.

    Syntax 

  2. return_statement ::= return [expression];

    Name Resolution Rules

  3. The expression, if any, of a return_statement is called the return expression. The result subtype of a function is the subtype denoted by the subtype_mark after the reserved word return in the profile of the function. The expected type for a return expression is the result type of the corresponding function.

    Legality Rules

  4. A return_statement shall be within a callable construct, and it applies to the innermost one. A return_statement shall not be within a body that is within the construct to which the return_statement applies.
  5. A function body shall contain at least one return_statement that applies to the function body, unless the function contains code_statements. A return_statement shall include a return expression if and only if it applies to a function body.

    Dynamic Semantics

  6. For the execution of a return_statement, the expression (if any) is first evaluated and converted to the result subtype.
  7. If the result type is class-wide, then the tag of the result is the tag of the value of the expression.
  8. If the result type is a specific tagged type:
    1. If it is limited, then a check is made that the tag of the value of the return expression identifies the result type. Constraint_Error is raised if this check fails.
    2. If it is nonlimited, then the tag of the result is that of the result type.
  1. A type is a return-by-reference type if it is a descendant of one of the following:
    1. a tagged limited type;
    2. a task or protected type;
    3. a nonprivate type with the reserved word limited in its declaration;
    4. a composite type with a subcomponent of a return-by-reference type;
    5. a private type whose full type is a return-by-reference type.
  1. If the result type is a return-by-reference type, then a check is made that the return expression is one of the following:
    1. a name that denotes an object view whose accessibility level is not deeper than that of the master that elaborated the function body; or
    2. a parenthesized expression or qualified_expression whose operand is one of these kinds of expressions.
  1. The exception Program_Error is raised if this check fails.
  2. For a function with a return-by-reference result type the result is returned by reference; that is, the function call denotes a constant view of the object associated with the value of the return expression. For any other function, the result is returned by copy; that is, the converted value is assigned into an anonymous constant created at the point of the return_statement, and the function call denotes that object.
  3. Finally, a transfer of control is performed which completes the execution of the callable construct to which the return_statement applies, and returns to the caller.

    Examples

  4. Examples of return statements: 
  5. return;
-- in a procedure body, entry_body, or accept_statement

return Key_Value(Last_Index);
-- in a function body

Overloading of Operators

  1. An operator is a function whose designator is an operator_symbol. Operators, like other functions, may be overloaded.

    Name Resolution Rules

  2. Each use of a unary or binary operator is equivalent to a function_call with function_prefix being the corresponding operator_symbol, and with (respectively) one or two positional actual parameters being the operand(s) of the operator (in order).

    Legality Rules

  3. The subprogram_specification of a unary or binary operator shall have one or two parameters, respectively. A generic function instantiation whose designator is an operator_symbol is only allowed if the specification of the generic function has the corresponding number of parameters.
  4. Default_expressions are not allowed for the parameters of an operator (whether the operator is declared with an explicit subprogram_specification or by a generic_instantiation).
  5. An explicit declaration of "/=" shall not have a result type of the predefined type Boolean.

    Static Semantics

  6. A declaration of "=" whose result type is Boolean implicitly declares a declaration of "/=" that gives the complementary result. NOTES
  7. (8) The operators "+" and "-" are both unary and binary operators, and hence may be overloaded with both one- and two-parameter functions.

    Examples

  8. Examples of user-defined operators: 
  9. function "+" (Left, Right : Matrix) return Matrix;
function "+" (Left, Right : Vector) return Vector;

--  assuming that A, B, and C are of the type Vector
--  the following two statements are equivalent:

A := B + C;
A := "+"(B, C);

Packages

  1. Packages are program units that allow the specification of groups of logically related entities. Typically, a package contains the declaration of a type (often a private type or private extension) along with the declarations of primitive subprograms of the type, which can be called from outside the package, while their inner workings remain hidden from outside users.

Package Specifications and Declarations

  1. A package is generally provided in two parts: a package_specification and a package_body. Every package has a package_specification, but not all packages have a package_body.

    Syntax 

  2. package_declaration ::= package_specification;

  3. package_specification ::=
   package defining_program_unit_name is
      {basic_declarative_item}
   [private
      {basic_declarative_item}]
   end [[parent_unit_name.]identifier]
    1. If an identifier or parent_unit_name.identifier appears at the end of a package_specification, then this sequence of lexical elements shall repeat the defining_program_unit_name.

Legality Rules

  1. A package_declaration or generic_package_declaration requires a completion (a body) if it contains any declarative_item that requires a completion, but whose completion is not in its package_specification.

    Static Semantics

  2. The first list of declarative_items of a package_specification of a package other than a generic formal package is called the visible part of the package. The optional list of declarative_items after the reserved word private (of any package_specification) is called the private part of the package. If the reserved word private does not appear, the package has an implicit empty private part.
  3. An entity declared in the private part of a package is visible only within the declarative region of the package itself (including any child units -- see section Compilation Units - Library Units. In contrast, expanded names denoting entities declared in the visible part can be used even outside the package; furthermore, direct visibility of such entities can be achieved by means of use_clauses, see section Selected Components, and section Use Clauses.

    Dynamic Semantics

  4. The elaboration of a package_declaration consists of the elaboration of its basic_declarative_items in the given order. NOTES
  5. (1) The visible part of a package contains all the information that another program unit is able to know about the package.
  6. (2) If a declaration occurs immediately within the specification of a package, and the declaration has a corresponding completion that is a body, then that body has to occur immediately within the body of the package.

    Examples

  7. Example of a package declaration: 
  8. package Rational_Numbers is

  9. type Rational is
   record
      Numerator   : Integer;
      Denominator : Positive;
   end record;

  10.    function "="(X,Y : Rational) return Boolean;

  11.    function "/"  (X,Y : Integer)  return Rational;
   --  to construct a rational number

  12.    function "+"  (X,Y : Rational) return Rational;
   function "-"  (X,Y : Rational) return Rational;
   function "*"  (X,Y : Rational) return Rational;
   function "/"  (X,Y : Rational) return Rational;
end Rational_Numbers;
  13. There are also many examples of package declarations in the predefined language environment, see section Predefined Language Environment (normative).

Package Bodies

  1. In contrast to the entities declared in the visible part of a package, the entities declared in the package_body are visible only within the package_body itself. As a consequence, a package with a package_body can be used for the construction of a group of related subprograms in which the logical operations available to clients are clearly isolated from the internal entities.

    Syntax 

  2. package_body ::=
   package body defining_program_unit_name is
      declarative_part
   [begin
      handled_sequence_of_statements]
   end [[parent_unit_name.]identifier];
    1. If an identifier or parent_unit_name.identifier appears at the end of a package_body, then this sequence of lexical elements shall repeat the defining_program_unit_name.

Legality Rules

  1. A package_body shall be the completion of a previous package_declaration or generic_package_declaration. A library package_declaration or library generic_package_declaration shall not have a body unless it requires a body; pragma Elaborate_Body can be used to require a library_unit_declaration to have a body, see section Elaboration Control, if it would not otherwise require one.

    Static Semantics

  2. In any package_body without statements there is an implicit null_statement. For any package_declaration without an explicit completion, there is an implicit package_body containing a single null_statement. For a noninstance, nonlibrary package, this body occurs at the end of the declarative_part of the innermost enclosing program unit or block_statement; if there are several such packages, the order of the implicit package_bodies is unspecified. (For an instance, the implicit package_body occurs at the place of the instantiation (see section Generic Instantiation). For a library package, the place is partially determined by the elaboration dependences, see section Program Structure and Compilation Issues.

    Dynamic Semantics

  3. For the elaboration of a nongeneric package_body, its declarative_part is first elaborated, and its handled_sequence_of_statements is then executed. NOTES
  4. (3) A variable declared in the body of a package is only visible within this body and, consequently, its value can only be changed within the package_body. In the absence of local tasks, the value of such a variable remains unchanged between calls issued from outside the package to subprograms declared in the visible part. The properties of such a variable are similar to those of a "static" variable of C.
  5. (4) The elaboration of the body of a subprogram explicitly declared in the visible part of a package is caused by the elaboration of the body of the package. Hence a call of such a subprogram by an outside program unit raises the exception Program_Error if the call takes place before the elaboration of the package_body, see section Declarative Parts.

    Examples

  6. Example of a package body, see section Package Specifications and Declarations 
  7. package body Rational_Numbers is

  8.    procedure Same_Denominator (X,Y : in out Rational) is
   begin
      --  reduces X and Y to the same denominator:
      ...
   end Same_Denominator;

  9.    function "="(X,Y : Rational) return Boolean is
      U : Rational := X;
      V : Rational := Y;
   begin
      Same_Denominator (U,V);
      return U.Numerator = V.Numerator;
   end "=";

  10.    function "/" (X,Y : Integer) return Rational is
   begin
      if Y > 0 then
         return (Numerator => X,  Denominator => Y);
      else
         return (Numerator => -X, Denominator => -Y);
      end if;
   end "/";

  11.    function "+" (X,Y : Rational) return Rational is ...  end "+";
   function "-" (X,Y : Rational) return Rational is ...  end "-";
   function "*" (X,Y : Rational) return Rational is ...  end "*";
   function "/" (X,Y : Rational) return Rational is ...  end "/";

  12. end Rational_Numbers;

Private Types and Private Extensions

  1. The declaration (in the visible part of a package) of a type as a private type or private extension serves to separate the characteristics that can be used directly by outside program units (that is, the logical properties) from other characteristics whose direct use is confined to the package (the details of the definition of the type itself). See section Type Extensions, for an overview of type extensions.

    Syntax 

  2. private_type_declaration ::=
   type defining_identifier [discriminant_part] is
     [[abstract] tagged] [limited] private;

  3. private_extension_declaration ::=
   type defining_identifier [discriminant_part] is
     [abstract] new ancestor_subtype_indication with private;

    Legality Rules

  4. A private_type_declaration or private_extension_declaration declares a partial view of the type; such a declaration is allowed only as a declarative_item of the visible part of a package, and it requires a completion, which shall be a full_type_declaration that occurs as a declarative_item of the private part of the package. The view of the type declared by the full_type_declaration is called the full view. A generic formal private type or a generic formal private extension is also a partial view.
  5. A type shall be completely defined before it is frozen, see section Completions of Declarations, and section Freezing Rules. Thus, neither the declaration of a variable of a partial view of a type, nor the creation by an allocator of an object of the partial view are allowed before the full declaration of the type. Similarly, before the full declaration, the name of the partial view cannot be used in a generic_instantiation or in a representation item.
  6. A private type is limited if its declaration includes the reserved word limited; a private extension is limited if its ancestor type is limited. If the partial view is nonlimited, then the full view shall be nonlimited. If a tagged partial view is limited, then the full view shall be limited. On the other hand, if an untagged partial view is limited, the full view may be limited or nonlimited.
  7. If the partial view is tagged, then the full view shall be tagged. On the other hand, if the partial view is untagged, then the full view may be tagged or untagged. In the case where the partial view is untagged and the full view is tagged, no derivatives of the partial view are allowed within the immediate scope of the partial view; derivatives of the full view are allowed.
  8. The ancestor subtype of a private_extension_declaration is the subtype defined by the ancestor_subtype_indication; the ancestor type shall be a specific tagged type. The full view of a private extension shall be derived (directly or indirectly) from the ancestor type. In addition to the places where Legality Rules normally apply, see section Generic Instantiation, the requirement that the ancestor be specific applies also in the private part of an instance of a generic unit.
  9. If the declaration of a partial view includes a known_discriminant_part, then the full_type_declaration shall have a fully conforming (explicit) known_discriminant_part, see section Conformance Rules. The ancestor subtype may be unconstrained; the parent subtype of the full view is required to be constrained, see section Discriminants.
  10. If a private extension inherits known discriminants from the ancestor subtype, then the full view shall also inherit its discriminants from the ancestor subtype, and the parent subtype of the full view shall be constrained if and only if the ancestor subtype is constrained.
  11. If a partial view has unknown discriminants, then the full_type_declaration may define a definite or an indefinite subtype, with or without discriminants.
  12. If a partial view has neither known nor unknown discriminants, then the full_type_declaration shall define a definite subtype.
  13. If the ancestor subtype of a private extension has constrained discriminants, then the parent subtype of the full view shall impose a statically matching constraint on those discriminants.

    Static Semantics

  14. A private_type_declaration declares a private type and its first subtype. Similarly, a private_extension_declaration declares a private extension and its first subtype.
  15. A declaration of a partial view and the corresponding full_type_declaration define two views of a single type. The declaration of a partial view together with the visible part define the operations that are available to outside program units; the declaration of the full view together with the private part define other operations whose direct use is possible only within the declarative region of the package itself. Moreover, within the scope of the declaration of the full view, the characteristics of the type are determined by the full view; in particular, within its scope, the full view determines the classes that include the type, which components, entries, and protected subprograms are visible, what attributes and other predefined operations are allowed, and whether the first subtype is static (see section Private Operations).
  16. A private extension inherits components (including discriminants unless there is a new discriminant_part specified) and user-defined primitive subprograms from its ancestor type, in the same way that a record extension inherits components and user-defined primitive subprograms from its parent type, see section Derived Types and Classes.

    Dynamic Semantics

  17. The elaboration of a private_type_declaration creates a partial view of a type. The elaboration of a private_extension_declaration elaborates the ancestor_subtype_indication, and creates a partial view of a type. NOTES
  18. (5) The partial view of a type as declared by a private_type_declaration is defined to be a composite view (in 3.2). The full view of the type might or might not be composite. A private extension is also composite, as is its full view.
  19. (6) Declaring a private type with an unknown_discriminant_part is a way of preventing clients from creating uninitialized objects of the type; they are then forced to initialize each object by calling some operation declared in the visible part of the package. If such a type is also limited, then no objects of the type can be declared outside the scope of the full_type_declaration, restricting all object creation to the package defining the type. This allows complete control over all storage allocation for the type. Objects of such a type can still be passed as parameters, however.
  20. (7) The ancestor type specified in a private_extension_declaration and the parent type specified in the corresponding declaration of a record extension given in the private part need not be the same -- the parent type of the full view can be any descendant of the ancestor type. In this case, for a primitive subprogram that is inherited from the ancestor type and not overridden, the formal parameter names and default expressions (if any) come from the corresponding primitive subprogram of the specified ancestor type, while the body comes from the corresponding primitive subprogram of the parent type of the full view (see section Dispatching Operations of Tagged Types).

    Examples

  21. Examples of private type declarations: 
  22. type Key is private;
type File_Name is limited private;
  23. Example of a private extension declaration: 
  24. type List is new Ada.Finalization.Controlled with private;

Private Operations

  1. For a type declared in the visible part of a package or generic package, certain operations on the type do not become visible until later in the package -- either in the private part or the body. Such private operations are available only inside the declarative region of the package or generic package.

    Static Semantics

  2. The predefined operators that exist for a given type are determined by the classes to which the type belongs. For example, an integer type has a predefined "+" operator. In most cases, the predefined operators of a type are declared immediately after the definition of the type; the exceptions are explained below. Inherited subprograms are also implicitly declared immediately after the definition of the type, except as stated below.
  3. For a composite type, the characteristics, see section Private Types and Private Extensions of the type are determined in part by the characteristics of its component types. At the place where the composite type is declared, the only characteristics of component types used are those characteristics visible at that place. If later within the immediate scope of the composite type additional characteristics become visible for a component type, then any corresponding characteristics become visible for the composite type. Any additional predefined operators are implicitly declared at that place.
  4. The corresponding rule applies to a type defined by a derived_type_definition, if there is a place within its immediate scope where additional characteristics of its parent type become visible.
  5. For example, an array type whose component type is limited private becomes nonlimited if the full view of the component type is nonlimited and visible at some later place within the immediate scope of the array type. In such a case, the predefined "=" operator is implicitly declared at that place, and assignment is allowed after that place.
  6. Inherited primitive subprograms follow a different rule. For a derived_type_definition, each inherited primitive subprogram is implicitly declared at the earliest place, if any, within the immediate scope of the type_declaration, but after the type_declaration, where the corresponding declaration from the parent is visible. If there is no such place, then the inherited subprogram is not declared at all. An inherited subprogram that is not declared at all cannot be named in a call and cannot be overridden, but for a tagged type, it is possible to dispatch to it.
  7. For a private_extension_declaration, each inherited subprogram is declared immediately after the private_extension_declaration if the corresponding declaration from the ancestor is visible at that place. Otherwise, the inherited subprogram is not declared for the private extension, though it might be for the full type.
  8. The Class attribute is defined for tagged subtypes in 3.9. In addition, for every subtype S of an untagged private type whose full view is tagged, the following attribute is defined:
  9. S'Class 
    Denotes the class-wide subtype corresponding to the full view
of S. This attribute is allowed only from the beginning of
the private part in which the full view is declared, until
the declaration of the full view. After the full view, the
Class attribute of the full view can be used.
    NOTES
  10. (8) Because a partial view and a full view are two different views of one and the same type, outside of the defining package the characteristics of the type are those defined by the visible part. Within these outside program units the type is just a private type or private extension, and any language rule that applies only to another class of types does not apply. The fact that the full declaration might implement a private type with a type of a particular class (for example, as an array type) is relevant only within the declarative region of the package itself including any child units.
  11. The consequences of this actual implementation are, however, valid everywhere. For example: any default initialization of components takes place; the attribute Size provides the size of the full view; finalization is still done for controlled components of the full view; task dependence rules still apply to components that are task objects.
  12. (9) Partial views provide assignment (unless the view is limited), membership tests, selected components for the selection of discriminants and inherited components, qualification, and explicit conversion.
  13. (10) For a subtype S of a partial view, S'Size is defined, see section Representation Attributes. For an object A of a partial view, the attributes A'Size and A'Address are defined, see section Representation Attributes. The Position, First_Bit, and Last_Bit attributes are also defined for discriminants and inherited components.

    Examples

  14. Example of a type with private operations: 
  15. package Key_Manager is
   type Key is private;
   Null_Key : constant Key;
   -- a deferred constant declaration, see section Deferred Constants
   procedure Get_Key(K : out Key);
   function "<" (X, Y : Key) return Boolean;
private
   type Key is new Natural;
   Null_Key : constant Key := Key'First;
end Key_Manager;

  16. package body Key_Manager is
   Last_Key : Key := Null_Key;
   procedure Get_Key(K : out Key) is
   begin
      Last_Key := Last_Key + 1;
      K := Last_Key;
   end Get_Key;

  17.    function "<" (X, Y : Key) return Boolean is
   begin
      return Natural(X) < Natural(Y);
   end "<";
end Key_Manager;
    NOTES
  18. (11) Notes on the example: Outside of the package Key_Manager, the operations available for objects of type Key include assignment, the comparison for equality or inequality, the procedure Get_Key and the operator "<"; they do not include other relational operators such as ">=", or arithmetic operators.
  19. The explicitly declared operator "<" hides the predefined operator "<" implicitly declared by the full_type_declaration. Within the body of the function, an explicit conversion of X and Y to the subtype Natural is necessary to invoke the "<" operator of the parent type. Alternatively, the result of the function could be written as not (X >= Y), since the operator ">=" is not redefined.
  20. The value of the variable Last_Key, declared in the package body, remains unchanged between calls of the procedure Get_Key (See also the NOTES of section Package Bodies)

Deferred Constants

  1. Deferred constant declarations may be used to declare constants in the visible part of a package, but with the value of the constant given in the private part. They may also be used to declare constants imported from other languages, see section Interface to Other Languages (normative).

    Legality Rules

  2. A deferred constant declaration is an object_declaration with the reserved word constant but no initialization expression. The constant declared by a deferred constant declaration is called a deferred constant. A deferred constant declaration requires a completion, which shall be a full constant declaration (called the full declaration of the deferred constant), or a pragma Import (see section Interface to Other Languages (normative)).
  3. A deferred constant declaration that is completed by a full constant declaration shall occur immediately within the visible part of a package_ specification. For this case, the following additional rules apply to the corresponding full declaration:
    1. The full declaration shall occur immediately within the private part of the same package;
    2. The deferred and full constants shall have the same type;
    3. If the subtype defined by the subtype_indication in the deferred declaration is constrained, then the subtype defined by the subtype_indication in the full declaration shall match it statically. On the other hand, if the subtype of the deferred constant is unconstrained, then the full declaration is still allowed to impose a constraint. The constant itself will be constrained, like all constants;
    4. If the deferred constant declaration includes the reserved word aliased, then the full declaration shall also.
  1. A deferred constant declaration that is completed by a pragma Import need not appear in the visible part of a package_specification, and has no full constant declaration.
  2. The completion of a deferred constant declaration shall occur before the constant is frozen, see section Deferred Constants.

    Dynamic Semantics

  3. The elaboration of a deferred constant declaration elaborates the subtype_indication or (only allowed in the case of an imported constant) the array_type_definition. NOTES
  4. (12) The full constant declaration for a deferred constant that is of a given private type or private extension is not allowed before the corresponding full_type_declaration. This is a consequence of the freezing rules for types, see section Freezing Rules.

    Examples

  5. Examples of deferred constant declarations: 
  6. Null_Key : constant Key;      --  see section Private Operations

  7. CPU_Identifier : constant String(1..8);
pragma Import(Assembler, CPU_Identifier, Link_Name => "CPU_ID");
--  see section Interfacing Pragmas

Limited Types

  1. A limited type is (a view of) a type for which the assignment operation is not allowed. A nonlimited type is a (view of a) type for which the assignment operation is allowed.

    Legality Rules

  2. If a tagged record type has any limited components, then the reserved word limited shall appear in its record_type_definition.

    Static Semantics

  3. A type is limited if it is a descendant of one of the following:
    1. a type with the reserved word limited in its definition;
    2. a task or protected type;
    3. a composite type with a limited component.
  1. Otherwise, the type is nonlimited.
  2. There are no predefined equality operators for a limited type. NOTES
  3. (13) The following are consequences of the rules for limited types:
    1. An initialization expression is not allowed in an object_declaration if the type of the object is limited.
    2. A default expression is not allowed in a component_declaration if the type of the record component is limited.
    3. An initialized allocator is not allowed if the designated type is limited.
    4. A generic formal parameter of mode in must not be of a limited type.
  1. (14) Aggregates are not available for a limited composite type. Concatenation is not available for a limited array type.
  2. (15) The rules do not exclude a default_expression for a formal parameter of a limited type; they do not exclude a deferred constant of a limited type if the full declaration of the constant is of a nonlimited type.
  3. (16) As illustrated in section Private Operations, an untagged limited type can become nonlimited under certain circumstances.

    Examples

  4. Example of a package with a limited type: 
  5. package IO_Package is
   type File_Name is limited private;

  6.    procedure Open (F : in out File_Name);
   procedure Close(F : in out File_Name);
   procedure Read (F : in File_Name; Item : out Integer);
   procedure Write(F : in File_Name; Item : in  Integer);
private
   type File_Name is
      limited record
         Internal_Name : Integer := 0;
      end record;
end IO_Package;

  7. package body IO_Package is
   Limit : constant := 200;
   type File_Descriptor is record  ...  end record;
   Directory : array (1 .. Limit) of File_Descriptor;
   ...
   procedure Open (F : in out File_Name) is  ...  end;
   procedure Close(F : in out File_Name) is  ...  end;
   procedure Read (F : in File_Name; Item : out Integer) is ... end;
   procedure Write(F : in File_Name; Item : in  Integer) is ... end;
begin
   ...
end IO_Package;
    NOTES
  8. (17) Notes on the example: In the example above, an outside subprogram making use of IO_Package may obtain a file name by calling Open and later use it in calls to Read and Write. Thus, outside the package, a file name obtained from Open acts as a kind of password; its internal properties (such as containing a numeric value) are not known and no other operations (such as addition or comparison of internal names) can be performed on a file name. Most importantly, clients of the package cannot make copies of objects of type File_Name.
  9. This example is characteristic of any case where complete control over the operations of a type is desired. Such packages serve a dual purpose. They prevent a user from making use of the internal structure of the type. They also implement the notion of an encapsulated data type where the only operations on the type are those given in the package specification.
  10. The fact that the full view of File_Name is explicitly declared limited means that parameter passing and function return will always be by reference, see section Formal Parameter Modes, and section Return Statements.

User-Defined Assignment and Finalization

  1. Three kinds of actions are fundamental to the manipulation of objects: initialization, finalization, and assignment. Every object is initialized, either explicitly or by default, after being created (for example, by an object_declaration or allocator). Every object is finalized before being destroyed (for example, by leaving a subprogram_body containing an object_declaration, or by a call to an instance of Unchecked_Deallocation). An assignment operation is used as part of assignment_statements, explicit initialization, parameter passing, and other operations.
  2. Default definitions for these three fundamental operations are provided by the language, but a controlled type gives the user additional control over parts of these operations. In particular, the user can define, for a controlled type, an Initialize procedure which is invoked immediately after the normal default initialization of a controlled object, a Finalize procedure which is invoked immediately before finalization of any of the components of a controlled object, and an Adjust procedure which is invoked as the last step of an assignment to a (nonlimited) controlled object.

    Static Semantics

  3. The following language-defined library package exists: 
  4. package Ada.Finalization is
    pragma Preelaborate(Finalization);

  5.     type Controlled is abstract tagged private;

  6.     procedure Initialize(Object : in out Controlled);
    procedure Adjust                 (Object : in out Controlled);
    procedure Finalize               (Object : in out Controlled);

  7.     type Limited_Controlled is abstract tagged limited private;

  8.     procedure Initialize(Object : in out Limited_Controlled);
    procedure Finalize  (Object : in out Limited_Controlled);
private
    ... -- not specified by the language
end Ada.Finalization;
  9. A controlled type is a descendant of Controlled or Limited_Controlled. The (default) implementations of Initialize, Adjust, and Finalize have no effect. The predefined "=" operator of type Controlled always returns True, since this operator is incorporated into the implementation of the predefined equality operator of types derived from Controlled, as explained in section Relational Operators and Membership Tests. The type Limited_Controlled is like Controlled, except that it is limited and it lacks the primitive subprogram Adjust.

    Dynamic Semantics

  10. During the elaboration of an object_declaration, for every controlled subcomponent of the object that is not assigned an initial value (as defined in 3.3.1), Initialize is called on that subcomponent. Similarly, if the object as a whole is controlled and is not assigned an initial value, Initialize is called on the object. The same applies to the evaluation of an allocator, as explained in section Allocators.
  11. For an extension_aggregate whose ancestor_part is a subtype_mark, Initialize is called on all controlled subcomponents of the ancestor part; if the type of the ancestor part is itself controlled, the Initialize procedure of the ancestor type is called, unless that Initialize procedure is abstract.
  12. Initialize and other initialization operations are done in an arbitrary order, except as follows. Initialize is applied to an object after initialization of its subcomponents, if any (including both implicit initialization and Initialize calls). If an object has a component with an access discriminant constrained by a per-object expression, Initialize is applied to this component after any components that do not have such discriminants. For an object with several components with such a discriminant, Initialize is applied to them in order of their component_declarations. For an allocator, any task activations follow all calls on Initialize.
  13. When a target object with any controlled parts is assigned a value, either when created or in a subsequent assignment_statement, the assignment operation proceeds as follows:
    1. The value of the target becomes the assigned value.
    2. The value of the target is adjusted.
  1. To adjust the value of a (nonlimited) composite object, the values of the components of the object are first adjusted in an arbitrary order, and then, if the object is controlled, Adjust is called. Adjusting the value of an elementary object has no effect, nor does adjusting the value of a composite object with no controlled parts.
  2. For an assignment_statement, after the name and expression have been evaluated, and any conversion (including constraint checking) has been done, an anonymous object is created, and the value is assigned into it; that is, the assignment operation is applied. (Assignment includes value adjustment.) The target of the assignment_statement is then finalized. The value of the anonymous object is then assigned into the target of the assignment_statement. Finally, the anonymous object is finalized. As explained below, the implementation may eliminate the intermediate anonymous object, so this description subsumes the one given in section Assignment Statements.

    Implementation Permissions

  3. An implementation is allowed to relax the above rules (for nonlimited controlled types) in the following ways:
    1. For an assignment_statement that assigns to an object the value of that same object, the implementation need not do anything.
    2. For an assignment_statement for a noncontrolled type, the implementation may finalize and assign each component of the variable separately (rather than finalizing the entire variable and assigning the entire new value) unless a discriminant of the variable is changed by the assignment.
    3. For an aggregate or function call whose value is assigned into a target object, the implementation need not create a separate anonymous object if it can safely create the value of the aggregate or function call directly in the target object. Similarly, for an assignment_statement, the implementation need not create an anonymous object if the value being assigned is the result of evaluating a name denoting an object (the source object) whose storage cannot overlap with the target. If the source object might overlap with the target object, then the implementation can avoid the need for an intermediary anonymous object by exercising one of the above permissions and perform the assignment one component at a time (for an overlapping array assignment), or not at all (for an assignment where the target and the source of the assignment are the same object). Even if an anonymous object is created, the implementation may move its value to the target object as part of the assignment without re-adjusting so long as the anonymous object has no aliased subcomponents.

Completion and Finalization

  1. This subclause defines completion and leaving of the execution of constructs and entities. A master is the execution of a construct that includes finalization of local objects after it is complete (and after waiting for any local tasks -- see section Task Dependence - Termination of Tasks.), but before leaving. Other constructs and entities are left immediately upon completion.

    Dynamic Semantics

  2. The execution of a construct or entity is complete when the end of that execution has been reached, or when a transfer of control, see section Simple and Compound Statements - Sequences of Statements, causes it to be abandoned. Completion due to reaching the end of execution, or due to the transfer of control of an exit_, return_, goto_, or requeue_statement or of the selection of a terminate_alternative is normal completion. Completion is abnormal otherwise -- when control is transferred out of a construct due to abort or the raising of an exception.
  3. After execution of a construct or entity is complete, it is left, meaning that execution continues with the next action, as defined for the execution that is taking place. Leaving an execution happens immediately after its completion, except in the case of a master: the execution of a task_body, a block_statement, a subprogram_body, an entry_body, or an accept_statement. A master is finalized after it is complete, and before it is left.
  4. For the finalization of a master, dependent tasks are first awaited, as explained in section Task Dependence - Termination of Tasks. Then each object whose accessibility level is the same as that of the master is finalized if the object was successfully initialized and still exists. These actions are performed whether the master is left by reaching the last statement or via a transfer of control. When a transfer of control causes completion of an execution, each included master is finalized in order, from innermost outward.
  5. For the finalization of an object:
    1. If the object is of an elementary type, finalization has no effect;
    2. If the object is of a controlled type, the Finalize procedure is called;
    3. If the object is of a protected type, the actions defined in section Protected Units and Protected Objects are performed;
    4. If the object is of a composite type, then after performing the above actions, if any, every component of the object is finalized in an arbitrary order, except as follows: if the object has a component with an access discriminant constrained by a per-object expression, this component is finalized before any components that do not have such discriminants; for an object with several components with such a discriminant, they are finalized in the reverse of the order of their component_declarations.
  1. Immediately before an instance of Unchecked_Deallocation reclaims the storage of an object, the object is finalized. If an instance of Unchecked_Deallocation is never applied to an object created by an allocator, the object will still exist when the corresponding master completes, and it will be finalized then.
  2. The order in which the finalization of a master performs finalization of objects is as follows: Objects created by declarations in the master are finalized in the reverse order of their creation. For objects that were created by allocators for an access type whose ultimate ancestor is declared in the master, this rule is applied as though each such object that still exists had been created in an arbitrary order at the first freezing point, see section Freezing Rules, of the ultimate ancestor type.
  3. The target of an assignment statement is finalized before copying in the new value, as explained in section User-Defined Assignment and Finalization.
  4. The anonymous objects created by function calls and by aggregates are finalized no later than the end of the innermost enclosing declarative_item or statement; if that is a compound_statement, they are finalized before starting the execution of any statement within the compound_statement.

    Bounded (Run-Time) Errors

  5. It is a bounded error for a call on Finalize or Adjust to propagate an exception. The possible consequences depend on what action invoked the Finalize or Adjust operation:
    1. For a Finalize invoked as part of an assignment_statement, Program_Error is raised at that point.
    2. For an Adjust invoked as part of an assignment operation, any other adjustments due to be performed are performed, and then Program_Error is raised.
    3. For a Finalize invoked as part of a call on an instance of Unchecked_Deallocation, any other finalizations due to be performed are performed, and then Program_Error is raised.
    4. For a Finalize invoked by the transfer of control of an exit_, return_, goto_, or requeue_statement, Program_Error is raised no earlier than after the finalization of the master being finalized when the exception occurred, and no later than the point where normal execution would have continued. Any other finalizations due to be performed up to that point are performed before raising Program_Error.
    5. For a Finalize invoked by a transfer of control that is due to raising an exception, any other finalizations due to be performed for the same master are performed; Program_Error is raised immediately after leaving the master.
    6. For a Finalize invoked by a transfer of control due to an abort or selection of a terminate alternative, the exception is ignored; any other finalizations due to be performed are performed.
    NOTES
  1. (18) The rules in section Program Structure and Compilation Issues, imply that immediately prior to partition termination, Finalize operations are applied to library-level controlled objects (including those created by allocators of library-level access types, except those already finalized). This occurs after waiting for library-level tasks to terminate.
  2. (19) A constant is only constant between its initialization and finalization. Both initialization and finalization are allowed to change the value of a constant.
  3. (20) Abort is deferred during certain operations related to controlled types, as explained in section Abort of a Task - Abort of a Sequence of Statements. Those rules prevent an abort from causing a controlled object to be left in an ill-defined state.
  4. (21) The Finalize procedure is called upon finalization of a controlled object, even if Finalize was called earlier, either explicitly or as part of an assignment; hence, if a controlled type is visibly controlled (implying that its Finalize primitive is directly callable), or is nonlimited (implying that assignment is allowed), its Finalize procedure should be designed to have no ill effect if it is applied a second time to the same object.

Visibility Rules

  1. The rules defining the scope of declarations and the rules defining which identifiers, character_literals, and operator_symbols are visible at (or from) various places in the text of the program are described in this section. The formulation of these rules uses the notion of a declarative region.
  2. As explained in section Declarations and Types, a declaration declares a view of an entity and associates a defining name with that view. The view comprises an identification of the viewed entity, and possibly additional properties. A usage name denotes a declaration. It also denotes the view declared by that declaration, and denotes the entity of that view. Thus, two different usage names might denote two different views of the same entity; in this case they denote the same entity.

Declarative Region

Static Semantics

  1. For each of the following constructs, there is a portion of the program text called its declarative region, within which nested declarations can occur:
    1. any declaration, other than that of an enumeration type, that is not a completion of a previous declaration;
    2. a block_statement;
    3. a loop_statement;
    4. an accept_statement;
    5. an exception_handler.
  1. The declarative region includes the text of the construct together with additional text determined (recursively), as follows:
    1. If a declaration is included, so is its completion, if any.
    2. If the declaration of a library unit (including Standard -- see section Compilation Units - Library Units, is included, so are the declarations of any child units (and their completions, by the previous rule). The child declarations occur after the declaration.
    3. If a body_stub is included, so is the corresponding subunit.
    4. If a type_declaration is included, then so is a corresponding record_representation_clause, if any.
  1. The declarative region of a declaration is also called the declarative region of any view or entity declared by the declaration.
  2. A declaration occurs immediately within a declarative region if this region is the innermost declarative region that encloses the declaration (the immediately enclosing declarative region), not counting the declarative region (if any) associated with the declaration itself.
  3. A declaration is local to a declarative region if the declaration occurs immediately within the declarative region. An entity is local to a declarative region if the entity is declared by a declaration that is local to the declarative region.
  4. A declaration is global to a declarative region if the declaration occurs immediately within another declarative region that encloses the declarative region. An entity is global to a declarative region if the entity is declared by a declaration that is global to the declarative region. NOTES
  5. (1) The children of a parent library unit are inside the parent's declarative region, even though they do not occur inside the parent's declaration or body. This implies that one can use (for example) "P.Q" to refer to a child of P whose defining name is Q, and that after "use P;" Q can refer (directly) to that child.
  6. (2) As explained above and in section Compilation Units - Library Units, all library units are descendants of Standard, and so are contained in the declarative region of Standard. They are not inside the declaration or body of Standard, but they are inside its declarative region.
  7. (3) For a declarative region that comes in multiple parts, the text of the declarative region does not contain any text that might appear between the parts. Thus, when a portion of a declarative region is said to extend from one place to another in the declarative region, the portion does not contain any text that might appear between the parts of the declarative region.

Scope of Declarations

  1. For each declaration, the language rules define a certain portion of the program text called the scope of the declaration. The scope of a declaration is also called the scope of any view or entity declared by the declaration. Within the scope of an entity, and only there, there are places where it is legal to refer to the declared entity. These places are defined by the rules of visibility and overloading.

    Static Semantics

  2. The immediate scope of a declaration is a portion of the declarative region immediately enclosing the declaration. The immediate scope starts at the beginning of the declaration, except in the case of an overloadable declaration, in which case the immediate scope starts just after the place where the profile of the callable entity is determined (which is at the end of the _specification for the callable entity, or at the end of the generic_instantiation if an instance). The immediate scope extends to the end of the declarative region, with the following exceptions:
    1. The immediate scope of a library_item includes only its semantic dependents.
    2. The immediate scope of a declaration in the private part of a library unit does not include the visible part of any public descendant of that library unit.
  1. The visible part of (a view of) an entity is a portion of the text of its declaration containing declarations that are visible from outside. The private part of (a view of) an entity that has a visible part contains all declarations within the declaration of (the view of) the entity, except those in the visible part; these are not visible from outside. Visible and private parts are defined only for these kinds of entities: callable entities, other program units, and composite types.
    1. The visible part of a view of a callable entity is its profile.
    2. The visible part of a composite type other than a task or protected type consists of the declarations of all components declared (explicitly or implicitly) within the type_declaration.
    3. The visible part of a generic unit includes the generic_formal_part. For a generic package, it also includes the first list of basic_declarative_items of the package_specification. For a generic subprogram, it also includes the profile.
    4. The visible part of a package, task unit, or protected unit consists of declarations in the program unit's declaration other than those following the reserved word private, if any; see section Package Specifications and Declarations, and section Formal Packages, for packages, see section Task Units and Task Objects, for task units, and section Protected Units and Protected Objects, for protected units.
  1. The scope of a declaration always contains the immediate scope of the declaration. In addition, for a given declaration that occurs immediately within the visible part of an outer declaration, or is a public child of an outer declaration, the scope of the given declaration extends to the end of the scope of the outer declaration, except that the scope of a library_item includes only its semantic dependents.
  2. The immediate scope of a declaration is also the immediate scope of the entity or view declared by the declaration. Similarly, the scope of a declaration is also the scope of the entity or view declared by the declaration. NOTES
  3. (4) There are notations for denoting visible declarations that are not directly visible. For example, parameter_specifications are in the visible part of a subprogram_declaration so that they can be used in named-notation calls appearing outside the called subprogram. For another example, declarations of the visible part of a package can be denoted by expanded names appearing outside the package, and can be made directly visible by a use_clause.

Visibility

  1. The visibility rules, given below, determine which declarations are visible and directly visible at each place within a program. The visibility rules apply to both explicit and implicit declarations.

    Static Semantics

  2. A declaration is defined to be directly visible at places where a name consisting of only an identifier or operator_symbol is sufficient to denote the declaration; that is, no selected_component notation or special context (such as preceding => in a named association) is necessary to denote the declaration. A declaration is defined to be visible wherever it is directly visible, as well as at other places where some name (such as a selected_component) can denote the declaration.
  3. The syntactic category direct_name is used to indicate contexts where direct visibility is required. The syntactic category selector_name is used to indicate contexts where visibility, but not direct visibility, is required.
  4. There are two kinds of direct visibility: immediate visibility and use-visibility. A declaration is immediately visible at a place if it is directly visible because the place is within its immediate scope. A declaration is use-visible if it is directly visible because of a use_clause, see section Use Clauses. Both conditions can apply.
  5. A declaration can be hidden, either from direct visibility, or from all visibility, within certain parts of its scope. Where hidden from all visibility, it is not visible at all (neither using a direct_name nor a selector_name). Where hidden from direct visibility, only direct visibility is lost; visibility using a selector_name is still possible.
  6. Two or more declarations are overloaded if they all have the same defining name and there is a place where they are all directly visible.
  7. The declarations of callable entities (including enumeration literals) are overloadable, meaning that overloading is allowed for them.
  8. Two declarations are homographs if they have the same defining name, and, if both are overloadable, their profiles are type conformant. An inner declaration hides any outer homograph from direct visibility.
  9. Two homographs are not generally allowed immediately within the same declarative region unless one overrides the other (see Legality Rules below). A declaration overrides another homograph that occurs immediately within the same declarative region in the following cases:
    1. An explicit declaration overrides an implicit declaration of a primitive subprogram, regardless of which declaration occurs first;
    2. The implicit declaration of an inherited operator overrides that of a predefined operator;
    3. An implicit declaration of an inherited subprogram overrides a previous implicit declaration of an inherited subprogram.
    4. For an implicit declaration of a primitive subprogram in a generic unit, there is a copy of this declaration in an instance. However, a whole new set of primitive subprograms is implicitly declared for each type declared within the visible part of the instance. These new declarations occur immediately after the type declaration, and override the copied ones. The copied ones can be called only from within the instance; the new ones can be called only from outside the instance, although for tagged types, the body of a new one can be executed by a call to an old one.
  1. A declaration is visible within its scope, except where hidden from all visibility, as follows:
    1. An overridden declaration is hidden from all visibility within the scope of the overriding declaration.
    2. A declaration is hidden from all visibility until the end of the declaration, except:
      1. For a record type or record extension, the declaration is hidden from all visibility only until the reserved word record;
      2. For a package_declaration, task declaration, protected declaration, generic_package_declaration, or subprogram_body, the declaration is hidden from all visibility only until the reserved word is of the declaration.
    1. If the completion of a declaration is a declaration, then within the scope of the completion, the first declaration is hidden from all visibility. Similarly, a discriminant_specification or parameter_specification is hidden within the scope of a corresponding discriminant_specification or parameter_specification of a corresponding completion, or of a corresponding accept_statement.
    2. The declaration of a library unit (including a library_unit_renaming_declaration) is hidden from all visibility except at places that are within its declarative region or within the scope of a with_clause that mentions it. For each declaration or renaming of a generic unit as a child of some parent generic package, there is a corresponding declaration nested immediately within each instance of the parent. Such a nested declaration is hidden from all visibility except at places that are within the scope of a with_clause that mentions the child.
  1. A declaration with a defining_identifier or defining_operator_symbol is immediately visible (and hence directly visible) within its immediate scope except where hidden from direct visibility, as follows:
    1. A declaration is hidden from direct visibility within the immediate scope of a homograph of the declaration, if the homograph occurs within an inner declarative region;
    2. A declaration is also hidden from direct visibility where hidden from all visibility.

Name Resolution Rules

  1. A direct_name shall resolve to denote a directly visible declaration whose defining name is the same as the direct_name. A selector_name shall resolve to denote a visible declaration whose defining name is the same as the selector_name.
  2. These rules on visibility and direct visibility do not apply in a context_clause, a parent_unit_name, or a pragma that appears at the place of a compilation_unit. For those contexts, see the rules in section Environment-Level Visibility Rules.

    Legality Rules

  3. An explicit declaration is illegal if there is a homograph occurring