Introduction

This is the Ada Reference Manual. 

Other available Ada documents include: 

Ada was originally designed with three overriding concerns: program reliability and maintenance, programming as a human activity, and efficiency. The 1995 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. Subsequent editions, including this fourth edition, have provided further flexibility and added more standardized packages within the framework provided by the 1995 revision.

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.

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.

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.

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. Parallel constructs were introduced to simplify making safe and efficient use of modern multicore architectures.

An Ada program is composed of one or more program units. Program units can 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 is visible to other units, and a body, containing the implementation details, which are not visible to other units. Most program units can be compiled separately.

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.

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 names the library units it requires.

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 can read data, update variables, or produce some output. It can 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.

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.

Subprogram and package units can 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.

A task unit is the basic unit for defining a task whose sequence of actions can be executed concurrently with those of other tasks. Such tasks can be implemented on multicomputers, multiprocessors, or with interleaved execution on a single processor. A task unit can define either a single executing task or a task type permitting the creation of any number of similar tasks.

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 blocks the execution of the body until it evaluates to True. A protected unit can define a single protected object or a protected type permitting the creation of several similar objects.

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.

The declarative part associates names with declared entities. For example, a name can 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.

The sequence of statements describes a sequence of actions to be performed. The statements are executed in succession (unless a transfer of control causes execution to continue from another place).

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.

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.

The loop statement provides the basic iterative mechanism in the language. A loop statement specifies a sequence of statements that are executed repeatedly as directed by an iteration scheme, or until an exit statement is encountered.

A block statement comprises a sequence of statements preceded by the declaration of local entities used by the statements.

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 can 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. Various parallel constructs, including parallel loops and parallel blocks, support the initiation of multiple logical threads of control designed to execute in parallel when multiple processors are available.

Execution of a program unit can encounter error situations in which normal program execution cannot continue. For example, an arithmetic computation can exceed the maximum allowed value of a number, or an attempt can 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.

Every object in the language has a type, which characterizes a set of values and a set of applicable operations. The main categories of types are elementary types (comprising enumeration, numeric, and access types) and composite types (including array and record types).

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, Wide_Character, and Wide_Wide_Character are predefined.

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.

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, Wide_String, and Wide_Wide_String are predefined.

Record, task, and protected types can 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.

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 can designate the same object, and components of one object can 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.

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.

From any type a new type can be defined by derivation. A type, together with its derivatives (both direct and indirect) form a derivation class. Class-wide operations can be defined that accept as a parameter an operand of any type in a derivation class. For record and private types, the derivatives can be extensions of the parent type. Types that support these object-oriented capabilities of class-wide operations and type extension are 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.

  Interface types provide abstract models from which other interfaces and types can be composed and derived. This provides a reliable form of multiple inheritance. Interface types can also be implemented by task types and protected types thereby enabling concurrent programming and inheritance to be merged.

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.

Aspect 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 are to 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.

  Aspect clauses can also be used to specify more abstract properties of program entities, such as the pre- and postconditions of a subprogram, or the invariant for a private type. Additional aspects are specifiable to allow user-defined types to use constructs of the language, such as literals, aggregates, or indexing, normally reserved for particular language-defined categories of types, such as numeric types, record types, or array types.

The predefined environment of the language provides for input-output and other capabilities 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.

  The predefined standard library packages provide facilities such as string manipulation, containers of various kinds (vectors, lists, maps, etc.), mathematical functions, random number generation, and access to the execution environment.

  The specialized annexes define further predefined library packages and facilities with emphasis on areas such as real-time scheduling, interrupt handling, distributed systems, numerical computation, and high-integrity systems.

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. 

Paragraphs 44 through 57 have been replaced and moved to the Foreword. 

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  Ada is the result of a collective effort to design a common language for programming large scale and real-time systems.

  The common high order language program began in 1974. The requirements of the United States Department of Defense were formalized in a series of documents which were extensively reviewed by the Services, industrial organizations, universities, and foreign military departments. The Ada language was designed in accordance with the final (1978) form of these requirements, embodied in the Steelman specification.

  The Ada design team was led by Jean D. Ichbiah and has included Bernd Krieg-Brueckner, Brian A. Wichmann, Henry F. Ledgard, Jean-Claude Heliard, Jean-Loup Gailly, Jean-Raymond Abrial, John G.P. Barnes, Mike Woodger, Olivier Roubine, Paul N. Hilfinger, and Robert Firth.

  At various stages of the project, several people closely associated with the design team made major contributions. They include J.B. Goodenough, R.F. Brender, M.W. Davis, G. Ferran, K. Lester, L. MacLaren, E. Morel, I.R. Nassi, I.C. Pyle, S.A. Schuman, and S.C. Vestal.

  Two parallel efforts that were started in the second phase of this design had a deep influence on the language. One was the development of a formal definition using denotational semantics, with the participation of V. Donzeau-Gouge, G. Kahn, and B. Lang. The other was the design of a test translator with the participation of K. Ripken, P. Boullier, P. Cadiou, J. Holden, J.F. Hueras, R.G. Lange, and D.T. Cornhill. The entire effort benefitted from the dedicated assistance of Lyn Churchill and Marion Myers, and the effective technical support of B. Gravem, W.L. Heimerdinger, and P. Cleve. H.G. Schmitz served as program manager.

  Over the five years spent on this project, several intense week-long design reviews were conducted, with the participation of P. Belmont, B. Brosgol, P. Cohen, R. Dewar, A. Evans, G. Fisher, H. Harte, A.L. Hisgen, P. Knueven, M. Kronental, N. Lomuto, E. Ploedereder, G. Seegmueller, V. Stenning, D. Taffs, and also F. Belz, R. Converse, K. Correll, A.N. Habermann, J. Sammet, S. Squires, J. Teller, P. Wegner, and P.R. Wetherall.

  Several persons had a constructive influence with their comments, criticisms and suggestions. They include P. Brinch Hansen, G. Goos, C.A.R. Hoare, Mark Rain, W.A. Wulf, and also E. Boebert, P. Bonnard, H. Clausen, M. Cox, G. Dismukes, R. Eachus, T. Froggatt, H. Ganzinger, C. Hewitt, S. Kamin, R. Kotler, O. Lecarme, J.A.N. Lee, J.L. Mansion, F. Minel, T. Phinney, J. Roehrich, V. Schneider, A. Singer, D. Slosberg, I.C. Wand, the reviewers of Ada-Europe, AdaTech, Afcet, those of the LMSC review team, and those of the Ada Tokyo Study Group.

  These reviews and comments, the numerous evaluation reports received at the end of the first and second phase, the nine hundred language issue reports and test and evaluation reports received from fifteen different countries during the third phase of the project, the thousands of comments received during the ANSI Canvass, and the on-going work of the IFIP Working Group 2.4 on system implementation languages and that of the Purdue Europe LTPL-E committee, all had a substantial influence on the final definition of Ada.

  The Military Departments and Agencies have provided a broad base of support including funding, extensive reviews, and countless individual contributions by the members of the High Order Language Working Group and other interested personnel. In particular, William A. Whitaker provided leadership for the program during the formative stages. David A. Fisher was responsible for the successful development and refinement of the language requirement documents that led to the Steelman specification.

   The Ada 83 language definition was developed by Cii Honeywell Bull and later Alsys, and by Honeywell Systems and Research Center, under contract to the United States Department of Defense. William E. Carlson and later Larry E. Druffel served as the technical representatives of the United States Government and effectively coordinated the efforts of all participants in the Ada program.

This Reference Manual 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.

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).

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. Källberg (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. Månsson (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). 

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.

Special thanks go to R. Mathis, Convenor of ISO/IEC JTC 1/SC 22 Working Group 9. 

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.

  The editor [R. Brukardt (USA)] would like to thank the many people whose hard work and assistance has made this update possible.

  Thanks go out to all of the members of the ISO/IEC JTC 1/SC 22/WG 9 Ada Rapporteur Group, whose work on creating and editing the wording corrections was critical to the entire process. Especially valuable contributions came from the chairman of the ARG, E. Ploedereder (Germany), who kept the process moving; J. Barnes (UK) and K. Ishihata (Japan), whose extremely detailed reviews kept the editor on his toes; G. Dismukes (USA), M. Kamrad (USA), P. Leroy (France), S. Michell (Canada), T. Taft (USA), J. Tokar (USA), and other members too numerous to mention.

  Special thanks go to R. Duff (USA) for his explanations of the previous system of formatting of these documents during the tedious conversion to more modern formats. Special thanks also go to the convenor of ISO/IEC JTC 1/SC 22/WG 9, J. Moore (USA), without whose help and support the Corrigendum and this consolidated reference manual would not have been possible.

  The editor [R. Brukardt (USA)] would like to thank the many people whose hard work and assistance has made this update possible.

  Thanks go out to all of the members of the ISO/IEC JTC 1/SC 22/WG 9 Ada Rapporteur Group, whose work on creating and editing the wording corrections was critical to the entire process. Especially valuable contributions came from the chairman of the ARG, P. Leroy (France), who kept the process on schedule; J. Barnes (UK) whose careful reviews found many typographical errors; T. Taft (USA), who always seemed to have a suggestion when we were stuck, and who also was usually able to provide the valuable service of explaining why things were as they are; S. Baird (USA), who found many obscure problems with the proposals; and A. Burns (UK), who pushed many of the real-time proposals to completion. Other ARG members who contributed were: R. Dewar (USA), G. Dismukes (USA), R. Duff (USA), K. Ishihata (Japan), S. Michell (Canada), E. Ploedereder (Germany), J.P. Rosen (France), E. Schonberg (USA), J. Tokar (USA), and T. Vardanega (Italy).

  Special thanks go to Ada-Europe and the Ada Resource Association, without whose help and support the Amendment and this consolidated reference manual would not have been possible. M. Heaney (USA) requires special thanks for his tireless work on the containers packages. Finally, special thanks go to the convenor of ISO/IEC JTC 1/SC 22/WG 9, J. Moore (USA), who guided the document through the standardization process.

  The editor [R. Brukardt (USA)] would like to thank the many people whose hard work and assistance has made this revision possible.

  Thanks go out to all of the members of the ISO/IEC JTC 1/SC 22/WG 9 Ada Rapporteur Group, whose work on creating and editing the wording changes was critical to the entire process. Especially valuable contributions came from the chairman of the ARG, E. Schonberg (USA), who guided the work; T. Taft (USA), whose insights broke many logjams, both in design and wording; J. Barnes (UK) whose careful reviews uncovered many editorial errors; S. Baird (USA), who repeatedly found obscure interactions with the proposals that the rest of us missed. Other ARG members who substantially contributed were: A. Burns (UK), J. Cousins (UK), R. Dewar (USA), G. Dismukes (USA), R. Duff (USA), P. Leroy (France), B. Moore (Canada), E. Ploedereder (Germany), J.P. Rosen (France), B. Thomas (USA), and T. Vardanega (Italy).

  Special thanks go to Ada-Europe and the Ada Resource Association, without whose help and support this third edition of the Ada Standard would not have been possible. A special mention has to go to A. Beneschan (USA) for his efforts in eliminating sloppiness in our wording. M. Heaney (USA) also deserves a mention for his efforts to improve the containers packages. Finally, special thanks go to the convenor of ISO/IEC JTC 1/SC 22/WG 9, J. Tokar (USA), who guided the document through the standardization process. 

   The editor [R. Brukardt (USA)] would like to thank the many people whose hard work and assistance has made this update possible.

   Thanks go out to all of the members of the ISO/IEC JTC 1/SC 22/WG 9 Ada Rapporteur Group, whose work on creating and editing the wording changes was critical to the entire process. Especially valuable contributions came from the chairman of the ARG, J. Cousins (UK), who guided the work; T. Taft (USA), who seems to have the ability to cut any Gordian knot we encounter in wording; J. Barnes (UK) who continues to be able to find editorial errors invisible to most; S. Baird (USA), who so frequently finds obscure interactions that we now have named such things for him. Other ARG members who substantially contributed were: A. Burns (UK), R. Dewar (USA), G. Dismukes (USA), R. Duff (USA), B. Moore (Canada), E. Ploedereder (Germany), J.P. Rosen (France), E. Schonberg (USA), and T. Vardanega (Italy).

   Finally, special thanks go to the convenor of ISO/IEC JTC 1/SC 22/WG 9, J. Tokar (USA), who guided the document through the standardization process. 

   The editor [R. Brukardt] would like to thank the many people whose hard work and assistance has made this revision possible.

   Thanks go out to all of the members of the ISO/IEC JTC 1/SC 22/WG 9 Ada Rapporteur Group, whose work in all steps of the process, from determining problems to address, reviewing feature designs, and creating and editing wording changes, was critical to the entire process. Especially valuable contributions came from the chairman of the ARG through June 2018, J. Cousins, who guided the work and ensured we followed defined procedures; his replacement as chairman, S. Baird, who ably powered through obstacles to complete the work while continuing to find obscure interactions; T. Taft, who often solved difficult problems that had stumped others; B. Moore, whose frequent suggestions for parallel constructs greatly improved the result. Other ARG members who substantially contributed were: R. Amiard, J. Barnes, A. Burns, A. Charlet, G. Dismukes, C. Dross, R. Duff, E. Fish, E. Ploedereder, J.P. Rosen, F. Schanda, E. Schonberg, J. Squirek, T. Vardanega, and R. Wai.

   Finally, special thanks go to the convenor of ISO/IEC JTC 1/SC 22/WG 9, P. Rogers, who guided the document through the standardization process. 

This document has been revised with the corrections specified in Technical Corrigendum 1 for Ada 2012 (which corresponds to ISO/IEC 8652:2012/COR.1:2016) and other changes specifically for this fourth edition. In addition, a variety of editorial errors have been corrected. 

Changes to the original 1995 version of the Ada Reference Manual can be identified by the version number following the paragraph number. Paragraphs with a version number of /1 were changed by Technical Corrigendum 1 for Ada 95 or were editorial corrections at that time, while paragraphs with a version number of /2 were changed by Amendment 1 or were more recent editorial corrections, and paragraphs with a version number of /3 were changed by the 2012 edition of the Reference Manual (including additional editorial corrections). Paragraphs with a version number of /4 were changed by Technical Corrigendum 1 for Ada 2012 or were editorial corrections at that time. Paragraphs with a version number of /5 were changed by the 2022 edition of the Reference Manual (including additional editorial corrections). Paragraphs not so marked are unchanged since the original 1995 edition of the Ada Reference Manual, and have the same paragraph numbers as in that edition. In addition, some versions of this document include revision bars near the paragraph numbers. Where paragraphs are inserted, the paragraph numbers are of the form pp.nn, where pp is the number of the preceding paragraph, and nn is an insertion number. For instance, the first paragraph inserted after paragraph 8 is numbered 8.1, the second paragraph inserted is numbered 8.2, and so on. Deleted paragraphs are indicated by the text This paragraph was deleted. Deleted paragraphs include empty paragraphs that were numbered in the 1995 edition of the Ada Reference Manual.