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1.1 Scope

danger

This Reference Manual output has not been verified, and may contain omissions or errors. Report any problems on the tracking issue

1/3

This Reference Manual specifies the form and meaning of programs written in Ada. Its purpose is to promote the portability of Ada programs to a variety of computing systems.

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1.1.1 Extent​

1

This Reference Manual specifies:

2
  • The form of a program written in Ada;
    • 3
  • The effect of translating and executing such a program;
    • 4
  • The manner in which program units may be combined to form Ada programs;
    • 5
  • The language-defined library units that a conforming implementation is required to supply;
    • 6/5
  • The permissible variations in conformance to the rules of this document , and the manner in which they are to be documented;
    • 7/5
  • Those violations of the requirements of this document that a conforming implementation is required to detect, and the effect of attempting to translate or execute a program containing such violations;
    • 8/5
  • Those violations of the requirements of this document that a conforming implementation is not required to detect.
9

This Reference Manual does not specify:

10
  • The means whereby a program written in Ada is transformed into object code executable by a processor;
    • 11
  • The means whereby translation or execution of programs is invoked and the executing units are controlled;
    • 12
  • The size or speed of the object code, or the relative execution speed of different language constructs;
    • 13
  • The form or contents of any listings produced by implementations; in particular, the form or contents of error or warning messages;
    • 14/5
  • The effect of unspecified execution;
    • 15
  • The size of a program or program unit that will exceed the capacity of a particular conforming implementation.

1.1.2 Structure​

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This document contains thirteen clauses, fifteen annexes, and an index.

1.a/3
discussion
What Ada 83 called a “chapter” and Ada 95 (and Ada 2005) called a “section” is called a “clause” in this Reference Manual. Similarly, what Ada 83 called a “section” and Ada 95 (and Ada 2005) called a “clause” is called a “subclause” in this Reference Manual. Confused yet? This terminology is out of our hands; it is (and was) forced by ever-changing ISO rules for drafting Reference Manuals.
2

The core of the Ada language consists of:

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78

The following Specialized Needs Annexes define features that are needed by certain application areas:

91516

The core language and the Specialized Needs Annexes are normative, except that the material in each of the items listed below is informative:

17
  • Text under a NOTES or Examples heading.
    • 18/3
  • Each subclause whose title starts with the word “Example” or “Examples”.
19

All implementations shall conform to the core language. In addition, an implementation may conform separately to one or more Specialized Needs Annexes.

20

The following Annexes are informative:

21
25.a
discussion
The idea of the Specialized Needs Annexes is that implementations can choose to target certain application areas. For example, an implementation specifically targeted to embedded machines might support the application-specific features for Real-time Systems, but not the application-specific features for Information Systems.
25.b
note
The Specialized Needs Annexes extend the core language only in ways that users, implementations, and standards bodies are allowed to extend the language; for example, via additional library units, attributes, representation items (see 13.1), pragmas, and constraints on semantic details that are left unspecified by the core language. Many implementations already provide much of the functionality defined by Specialized Needs Annexes; our goal is to increase uniformity among implementations by defining standard ways of providing the functionality.
25.c/2
note
We recommend that the certification procedures allow implementations to certify the core language, plus any set of the Specialized Needs Annexes. We recommend that implementations not be allowed to certify a portion of one of the Specialized Needs Annexes, although implementations can, of course, provide uncertified support for such portions. We have designed the Specialized Needs Annexes assuming that this recommendation is followed. Thus, our decisions about what to include and what not to include in those annexes are based on the assumption that each annex is certified in an “all-or-nothing” manner.
25.d
note
An implementation may, of course, support extensions that are different from (but possibly related to) those defined by one of the Specialized Needs Annexes. We recommend that, where appropriate, implementations do this by adding library units that are children of existing language-defined library packages.
25.e
note
An implementation should not provide extensions that conflict with those defined in the Specialized Needs Annexes, in the following sense: Suppose an implementation supports a certain error-free program that uses only functionality defined in the core and in the Specialized Needs Annexes. The implementation should ensure that that program will still be error free in some possible full implementation of all of the Specialized Needs Annexes, and that the semantics of the program will not change. For example, an implementation should not provide a package with the same name as one defined in one of the Specialized Needs Annexes, but that behaves differently, even if that implementation does not claim conformance to that Annex.
25.f
note
Note that the Specialized Needs Annexes do not conflict with each other; it is the intent that a single implementation can conform to all of them.
26/4

Each clause is divided into subclauses that have a common structure. Each clause and subclause first introduces its subject. After the introductory text, text is labeled with the following headings:

Language Design Principles​

26.a
note
These are not rules of the language, but guiding principles or goals used in defining the rules of the language. In some cases, the goal is only partially met; such cases are explained.
26.b/3
note
This is not part of the definition of the language, and does not appear in the Ada 2022 RM.

Syntax​

27

Syntax rules (indented).

Name Resolution Rules​

28/3

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

28.a
discussion
These rules are observed at compile time. (We say “observed” rather than “checked”, because these rules are not individually checked. They are really just part of the Legality Rules in Clause 8 that require exactly one interpretation of each constituent of a complete context.) The only rules used in overload resolution are the Syntax Rules and the Name Resolution Rules.
28.b
note
When dealing with nonoverloadable declarations it sometimes makes no semantic difference whether a given rule is a Name Resolution Rule or a Legality Rule, and it is sometimes difficult to decide which it should be. We generally make a given rule a Name Resolution Rule only if it has to be. For example, “The name, if any, in a raise_statement shall be the name of an exception.” is under “Legality Rules.”

Legality Rules​

29

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

29.a
discussion
These rules are not used in overload resolution.
29.b
note
Note that run-time errors are always attached to exceptions; for example, it is not “illegal” to divide by zero, it just raises an exception.

Static Semantics​

30

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

30.a
discussion
The most important compile-time effects represent the effects on the symbol table associated with declarations (implicit or explicit). In addition, we use this heading as a bit of a grab bag for equivalences, package specifications, etc. For example, this is where we put statements like so-and-so is equivalent to such-and-such. (We ought to try to really mean it when we say such things!) Similarly, statements about magically-generated implicit declarations go here. These rules are generally written as statements of fact about the semantics, rather than as a you-shall-do-such-and-such sort of thing.

Post-Compilation Rules​

31

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.

31.a
discussion
It is not specified exactly when these rules are checked, so long as they are checked for any given partition before that partition starts running. An implementation may choose to check some such rules at compile time, and reject compilation_units accordingly. Alternatively, an implementation may check such rules when the partition is created (usually known as “link time”), or when the partition is mapped to a particular piece of hardware (but before the partition starts running).

Dynamic Semantics​

32

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

32.a
discussion
This heading describes what happens at run time. runtime checks, which raise exceptions upon failure, are described here. Each item that involves a runtime check is marked with the name of the check — these are the same check names that are used in a pragma Suppress. Principle: Every check should have a name, usable in a pragma Suppress.

Bounded (Run-Time) Errors​

33

Situations that result in bounded (run-time) errors (see 1.1.5).

33.a
discussion
The “bounds” of each such error are described here — that is, we characterize the set of all possible behaviors that can result from a bounded error occurring at run time.

Erroneous Execution​

34

Situations that result in erroneous execution (see 1.1.5).

Implementation Requirements​

35

Additional requirements for conforming implementations.

35.a
discussion
...as opposed to rules imposed on the programmer. An example might be, “The smallest representable duration, Duration'Small, shall not be greater than twenty milliseconds”.
35.b
note
It's really just an issue of how the rule is worded. We could write the same rule as “The smallest representable duration is an implementation-defined value less than or equal to 20 milliseconds” and then it would be under “Static Semantics”.

Documentation Requirements​

36

Documentation requirements for conforming implementations.

36.a
discussion
These requirements are beyond those that are implicitly specified by the phrase “implementation defined”. The latter require documentation as well, but we don't repeat these cases under this heading. Usually this heading is used for when the description of the documentation requirement is longer and does not correspond directly to one, narrow normative sentence.

Metrics​

37

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

Implementation Permissions​

38

Additional permissions given to the implementer.

38.a
discussion
For example, “The implementation is allowed to impose further restrictions on the record aggregates allowed in code statements.” When there are restrictions on the permission, those restrictions are given here also. For example, “An implementation is allowed to restrict the kinds of subprograms that are allowed to be main subprograms. However, it shall support at least parameterless procedures.” — we don't split this up between here and “Implementation Requirements”.

Implementation Advice​

39

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.

39.a/2
implementation advice
Whether or not each recommendation given in is followed — see M.3, “Implementation Advice” for a listing.
39.b/1
discussion
The advice generally shows the intended implementation, but the implementer is free to ignore it. The implementer is the sole arbiter of whether or not the advice has been obeyed, if not, whether the reason is a good one, and whether the required documentation is sufficient. It would be wrong for the ACATS to enforce any of this advice.
39.c
note
For example, “Whenever possible, the implementation should choose a value no greater than fifty microseconds for the smallest representable duration, Duration'Small.”
39.d
note
We use this heading, for example, when the rule is so low level or implementation-oriented as to be untestable. We also use this heading when we wish to encourage implementations to behave in a certain way in most cases, but we do not wish to burden implementations by requiring the behavior.
40
note
NOTE 1 Notes emphasize consequences of the rules described in the (sub)clause or elsewhere. This material is informative.

Examples​

41

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

41.a.1/5
discussion
Names used in examples refer either to language-defined entities or to entities declared in other Examples in this Reference Manual. In the latter case, if the reference is to a later example, we use a cross-reference (-- See <<some clause>>.) to show the source of the declaration. In theory, one could combine all of the examples (excepting examples of fragments like identifiers or type_definitions) in an appropriate unit and compile them all.
41.a
note


The next three headings list all language changes between Ada 83 and Ada 95. Language changes are any change that changes the set of text strings that are legal Ada programs, or changes the meaning of any legal program. Wording changes, such as changes in terminology, are not language changes. Each language change falls into one of the following three categories:

Inconsistencies With Ada 83​

41.b
note
This heading lists all of the upward inconsistencies between Ada 83 and Ada 95. Upward inconsistencies are situations in which a legal Ada 83 program is a legal Ada 95 program with different semantics. This type of upward incompatibility is the worst type for users, so we only tolerate it in rare situations.
41.c
note
(Note that the semantics of a program is not the same thing as the behavior of the program. Because of Ada's indeterminacy, the “semantics” of a given feature describes a set of behaviors that can be exhibited by that feature. The set can contain more than one allowed behavior. Thus, when we ask whether the semantics changes, we are asking whether the set of behaviors changes.)
41.d/4
note
This is not part of the definition of the language, and does not appear in the Ada 95, Ada 2005, Ada 2012, or Ada 2022 RM.

Incompatibilities With Ada 83​

41.e
note
This heading lists all of the upward incompatibilities between Ada 83 and Ada 95, except for the ones listed under “Inconsistencies With Ada 83” above. These are the situations in which a legal Ada 83 program is illegal in Ada 95. We do not generally consider a change that turns erroneous execution into an exception, or into an illegality, to be upwardly incompatible.
41.f/4
note
This is not part of the definition of the language, and does not appear in the Ada 95, Ada 2005, Ada 2012, or Ada 2022 RM.

Extensions to Ada 83​

41.g
note
This heading is used to list all upward compatible language changes; that is, language extensions. These are the situations in which a legal Ada 95 program is not a legal Ada 83 program. The vast majority of language changes fall into this category.
41.h/4
note
This is not part of the definition of the language, and does not appear in the Ada 95, Ada 2005, Ada 2012, or Ada 2022 RM.
41.i
note


As explained above, the next heading does not represent any language change:

Wording Changes from Ada 83​

41.j/2
note
This heading lists some of the nonsemantic changes between the Ada 83 RM and the Ada 95 RM. It is incomplete; we have not attempted to list all wording changes, but only the “interesting” ones.
41.k/4
note
This is not part of the definition of the language, and does not appear in the Ada 95, Ada 2005, Ada 2012, or Ada 2022 RM.
41.l/2
discussion


The next three headings list all language changes between Ada 95 and Ada 2005 (the language defined by the Ada 95 standard plus Technical Corrigendum 1 plus Amendment 1). Each language change falls into one of the following three categories:

Inconsistencies With Ada 95​

41.m/2
note
This heading lists all of the upward inconsistencies between Ada 95 and Ada 2005. Upward inconsistencies are situations in which a legal Ada 95 program is a legal Ada 2005 program with different semantics.
41.n/3
correction
Inconsistencies marked with Corrigendum: are corrections to the original Ada 95 definition introduced by Corrigendum 1. Inconsistencies marked with Amendment are corrections to the original Ada 95 definition added by Amendment 1. Formally, these are inconsistencies caused by Ada Issues classified as Binding Interpretations; implementations of Ada 95 are supposed to follow these corrections, not the original flawed language definition. Thus, these strictly speaking are not inconsistencies between Ada 95 and Ada 2005. Practically, however, they very well may be, as early Ada 95 implementations might not follow the recommendation. Inconsistencies so marked are not portable between Ada 95 implementations, while usually Ada 2005 will have more clearly defined behavior. Therefore, we document these for completeness.
41.o/4
note
This is not part of the definition of the language, and does not appear in the Ada 2005 , Ada 2012, or Ada 2022 RM.

Incompatibilities With Ada 95​

41.p/2
note
This heading lists all of the upward incompatibilities between Ada 95 and Ada 2005, except for the ones listed under “Inconsistencies With Ada 95” above. These are the situations in which a legal Ada 95 program is illegal in Ada 2005.
41.q/3
correction
As with inconsistencies, incompatibilities marked with Corrigendum: are corrections to the original Ada 95 definition introduced by Corrigendum 1. Incompatibilities marked with Amendment are corrections to the original Ada 95 definition added by Amendment 1. Formally, these are incompatibilities caused by Ada Issues classified as Binding Interpretations; implementations of Ada 95 are supposed to follow these corrections, not the original flawed language definition. Thus, these strictly speaking are not incompatibilities between Ada 95 and Ada 2005. Practically, however, they very well may be, as early Ada 95 implementations might not follow the recommendation. Therefore, some Ada 95 implementations may be able to compile the examples, while others might not. In contrast, Ada 2005 compilers will have consistent behavior. Therefore, we document these for completeness.
41.r/4
note
This is not part of the definition of the language, and does not appear in the Ada 2005 , Ada 2012, or Ada 2022 RM.

Extensions to Ada 95​

41.s/2
note
This heading is used to list all upward compatible language changes; that is, language extensions. These are the situations in which a legal Ada 2005 program is not a legal Ada 95 program. The vast majority of language changes fall into this category.
41.t/3
correction
As with incompatibilities, extensions marked with Corrigendum: are corrections to the original Ada 95 definition introduced by Corrigendum 1. Extensions marked with Amendment are corrections to the original Ada 95 definition added by Amendment 1. Formally, these are extensions allowed by Ada Issues classified as Binding Interpretations. As corrections, implementations of Ada 95 are allowed to implement these extensions. Thus, these strictly speaking are not extensions of Ada 95; they're part of Ada 95. Practically, however, they very well may be extensions, as early Ada 95 implementations might not implement the extension. Therefore, some Ada 95 implementations may be able to compile the examples, while others might not. In contrast, Ada 2005 compilers will always support the extensions. Therefore, we document these for completeness.
41.u/4
note
This is not part of the definition of the language, and does not appear in the Ada 2005 , Ada 2012, or Ada 2022 RM.
41.v/2
note


As explained above, the next heading does not represent any language change:

Wording Changes from Ada 95​

41.w/2
note
This heading lists some of the nonsemantic changes between the Ada 95 RM and the Ada 2005 RM. This heading lists only “interesting” changes (for instance, editorial corrections are not listed). Changes which come from Technical Corrigendum 1 are marked Corrigendum; unmarked changes come from Amendment 1.
41.x/4
note
This is not part of the definition of the language, and does not appear in the Ada 2005 , Ada 2012, or Ada 2022 RM.
41.y/3
discussion


The next three headings list all language changes between Ada 2005 (the language defined by the Ada 95 standard plus Technical Corrigendum 1 plus Amendment 1) and Ada 2012 (the language defined by the third edition of the Reference Manual). Each language change falls into one of the following three categories:

Inconsistencies With Ada 2005​

41.z/3
note
This heading lists all of the upward inconsistencies between Ada 2005 and Ada 2012. Upward inconsistencies are situations in which a legal Ada 2005 program is a legal Ada 2012 program with different semantics.
41.aa/3
correction
Inconsistencies marked with are corrections to the original Ada 2005 definition added by the third edition of the Reference Manual. Formally, these are inconsistencies caused by Ada Issues classified as Binding Interpretations; implementations of Ada 2005 are supposed to follow these corrections, not the original flawed language definition. Thus, these strictly speaking are not inconsistencies between Ada 2005 and Ada 2012. Practically, however, they very well may be, as early Ada 2005 implementations might not follow the recommendation. Inconsistencies so marked are not portable between Ada 2005 implementations, while usually Ada 2012 will have more clearly defined behavior. Therefore, we document these for completeness.
41.bb/4
note
This is not part of the definition of the language, and does not appear in the Ada 2012 or Ada 2022 RM.

Incompatibilities With Ada 2005​

41.cc/3
note
This heading lists all of the upward incompatibilities between Ada 2005 and Ada 2012, except for the ones listed under “Inconsistencies With Ada 2005” above. These are the situations in which a legal Ada 2005 program is illegal in Ada 2012.
41.dd/3
correction
As with inconsistencies, incompatibilities marked with are corrections to the original Ada 2005 definition added by the third edition. Formally, these are incompatibilities caused by Ada Issues classified as Binding Interpretations; implementations of Ada 2005 are supposed to follow these corrections, not the original flawed language definition. Thus, these strictly speaking are not incompatibilities between Ada 2005 and Ada 2012. Practically, however, they very well may be, as early Ada 2005 implementations might not follow the recommendation. Therefore, some Ada 2005 implementations may be able to compile the examples, while others might not. In contrast, Ada 2012 compilers will have consistent behavior. Therefore, we document these for completeness.
41.ee/4
note
This is not part of the definition of the language, and does not appear in the Ada 2012 or Ada 2022 RM.

Extensions to Ada 2005​

41.ff/3
note
This heading is used to list all upward compatible language changes; that is, language extensions. These are the situations in which a legal Ada 2012 program is not a legal Ada 2005 program. The vast majority of language changes fall into this category.
41.gg/3
correction
As with incompatibilities, extensions marked with are corrections to the original Ada 2005 definition added by the third edition. Formally, these are extensions allowed by Ada Issues classified as Binding Interpretations. As corrections, implementations of Ada 2005 (and sometimes Ada 95) are allowed to implement these extensions. Thus, these strictly speaking are not extensions of Ada 2005; they're part of Ada 2005. Practically, however, they very well may be extensions, as early Ada 2005 implementations might not implement the extension. Therefore, some Ada 2005 implementations may be able to compile the examples, while others might not. In contrast, Ada 2012 compilers will always support the extensions. Therefore, we document these for completeness.
41.hh/4
note
This is not part of the definition of the language, and does not appear in the Ada 2012 or Ada 2022 RM.
41.ii/3
note


As explained above, the next heading does not represent any language change:

Wording Changes from Ada 2005​

41.jj/3
correction
This heading lists some of the nonsemantic changes between the Ada 2005 RM and the Ada 2012 RM. This heading lists only “interesting” changes (for instance, editorial corrections are not listed). Items marked come from Ada Issues classified as Binding Interpretations and strictly speaking belong to Ada 2005; other items only belong to Ada 2012.
41.kk/4
note
This is not part of the definition of the language, and does not appear in the Ada 2012 or Ada 2022 RM.
41.ll/5
discussion


The next three headings list all language changes between Ada 2012 (the language defined by ISO/IEC 8652:2012(E)) and Ada 2022 (the language defined by this edition of the Reference Manual) . Each language change falls into one of the following three categories:

Inconsistencies With Ada 2012​

41.mm/5
note
This heading lists all of the upward inconsistencies between Ada 2012 and Ada 2022 . Upward inconsistencies are situations in which a legal Ada 2012 program is a legal Ada 2022 program with different semantics.
41.nn/5
correction
Inconsistencies marked with Corrigendum: are corrections to the original Ada 2012 definition added by Technical Corrigendum 1 to Ada 2012. Inconsistencies marked with are corrections to the original Ada 2012 definition added by this edition of the Reference Manual. Formally, these are inconsistencies caused by Ada Issues classified as Binding Interpretations; implementations of Ada 2012 are supposed to follow these corrections, not the original flawed language definition. Thus, these strictly speaking are not inconsistencies between Ada 2012 and Ada 2022 . Practically, however, they very well may be, as early Ada 2012 implementations might not follow the recommendation. Inconsistencies so marked are not portable between Ada 2012 implementations, while usually Ada 2022 will have more clearly defined behavior. Therefore, we document these for completeness.
41.oo/4
note
This is not part of the definition of the language, and does not appear in any version of the Ada RM.

Incompatibilities With Ada 2012​

41.pp/5
note
This heading lists all of the upward incompatibilities between Ada 2012 and Ada 2022 , except for the ones listed under “Inconsistencies With Ada 2012” above. These are the situations in which a legal Ada 2012 program is illegal in Ada 2022 .
41.qq/5
correction
As with inconsistencies, incompatibilities marked with Corrigendum: are corrections to the original Ada 2012 definition added by Corrigendum 1 to Ada 2012, and incompatibilities marked with are corrections to the original Ada 2012 definition added by this edition. Formally, these are incompatibilities caused by Ada Issues classified as Binding Interpretations; implementations of Ada 2012 are supposed to follow these corrections, not the original flawed language definition. Thus, these strictly speaking are not incompatibilities between Ada 2012 and Ada 2022 . Practically, however, they very well may be, as early Ada 2012 implementations might not follow the recommendation. Therefore, some Ada 2012 implementations may be able to compile the examples, while others might not. In contrast, Ada 2022 compilers will have consistent behavior. Therefore, we document these for completeness.
41.rr/4
note
This is not part of the definition of the language, and does not appear in any version of the Ada RM.

Extensions to Ada 2012​

41.ss/5
note
This heading is used to list all upward compatible language changes; that is, language extensions. These are the situations in which a legal Ada 2022 program is not a legal Ada 2012 program. The vast majority of language changes fall into this category.
41.tt/5
correction
As with incompatibilities, extensions marked with Corrigendum: are corrections to the original Ada 2012 definition added by Corrigendum 1 to Ada 2012, and extensions marked with are corrections to the original Ada 2012 definition added by this edition. Formally, these are extensions allowed by Ada Issues classified as Binding Interpretations. As corrections, implementations of Ada 2012 (and sometimes Ada 95) are allowed to implement these extensions. Thus, these strictly speaking are not extensions of Ada 2012; they're part of Ada 2012. Practically, however, they very well may be extensions, as early Ada 2012 implementations might not implement the extension. Therefore, some Ada 2012 implementations may be able to compile the examples, while others might not. In contrast, Ada 2022 compilers will always support the extensions. Therefore, we document these for completeness.
41.uu/4
note
This is not part of the definition of the language, and does not appear in any version of the Ada RM.
41.vv/4
note


As explained above, the next heading does not represent any language change:

Wording Changes from Ada 2012​

41.ww/5
correction
This heading lists some of the nonsemantic changes between the Ada 2012 RM and the Ada 2022 RM . This heading lists only “interesting” changes (for instance, editorial corrections are not listed). Items marked Corrigendum: are included in Corrigendum 1 to Ada 2012, and items marked with are added by this edition. In both cases, the items come from Ada Issues classified as Binding Interpretations and strictly speaking belong to Ada 2012; other items only belong to Ada 2022.
41.xx/4
note
This is not part of the definition of the language, and does not appear in any version of the Ada RM.

1.1.3 Conformity of an Implementation​

Implementation Requirements​

1

A conforming implementation shall:

1.a
discussion
The implementation is the software and hardware that implements the language. This includes compiler, linker, operating system, hardware, etc.
1.b
note
We first define what it means to “conform” in general — basically, the implementation has to properly implement the normative rules given throughout the Reference Manual. Then we define what it means to conform to a Specialized Needs Annex — the implementation must support the core features plus the features of that Annex. Finally, we define what it means to “conform to the Reference Manual” — this requires support for the core language, and allows partial (but not conflicting) support for the Specialized Needs Annexes.
2
  • Translate and correctly execute legal programs written in Ada, provided that they are not so large as to exceed the capacity of the implementation;
    • 3
  • 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.a
implementation defined
Capacity limitations of the implementation.
4
  • Identify all programs or program units that contain errors whose detection is required by this Reference Manual;
4.a
discussion
Note that we no longer use the term “rejection” of programs or program units. We require that programs or program units with errors or that exceed some capacity limit be “identified”. The way in which errors or capacity problems are reported is not specified.
4.b
note
An implementation is allowed to use standard error-recovery techniques. We do not disallow such techniques from being used across compilation_unit or compilation boundaries.
4.c
note
See also the Implementation Requirements of 10.2, which disallow the execution of illegal partitions.
5
  • Supply all language-defined library units required by this Reference Manual;
5.a
implementation note
An implementation cannot add to or modify the visible part of a language-defined library unit, except where such permission is explicitly granted, unless such modifications are semantically neutral with respect to the client compilation units of the library unit. An implementation defines the contents of the private part and body of language-defined library units.
5.b
note
An implementation can add with_clauses and use_clauses, since these modifications are semantically neutral to clients. (The implementation might need with_clauses in order to implement the private part, for example.) Similarly, an implementation can add a private part even in cases where a private part is not shown in the Reference Manual. Explicit declarations can be provided implicitly or by renaming, provided the changes are semantically neutral.
5.c
note
Wherever in the Reference Manual the text of a language-defined library unit contains an italicized phrase starting with “implementation-defined”, the implementation's version will replace that phrase with some implementation-defined text that is syntactically legal at that place, and follows any other applicable rules.
5.d
note
Note that modifications are permitted, even if there are other tools in the environment that can detect the changes (such as a program library browser), so long as the modifications make no difference with respect to the static or dynamic semantics of the resulting programs, as defined by the Reference Manual.
6
  • Contain no variations except those explicitly permitted by this Reference Manual, or those that are impossible or impractical to avoid given the implementation's execution environment;
6.a
implementation defined
Variations from the standard that are impractical to avoid given the implementation's execution environment.
6.b
reason
The “impossible or impractical” wording comes from AI-325. It takes some judgement and common sense to interpret this. Restricting compilation units to less than 4 lines is probably unreasonable, whereas restricting them to less than 4 billion lines is probably reasonable (at least given today's technology). We do not know exactly where to draw the line, so we have to make the rule vague.
7
  • Specify all such variations in the manner prescribed by this Reference Manual.
8

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:

9
10.a
implementation defined
Which code_statements cause external interactions.
11
  • Any call on an imported subprogram (see Annex B), including any parameters passed to it;
    • 12
  • Any result returned or exception propagated from a main subprogram (see 10.2) or an exported subprogram (see Annex B) to an external caller;
12.a
discussion
By “result returned” we mean to include function results and values returned in [in] out parameters.
12.a.1/1
note
{8652/0094} The lack of a result from a program that does not terminate is also included here.
13
  • [Any read or update of an atomic or volatile object (see C.6);]
    • 14
  • The values of imported and exported objects (see Annex B) at the time of any other interaction with the external environment.
14.a/3
note
To be honest: Also other uses of imported and exported entities, as defined by the implementation, if the implementation supports such importing or exporting.
15

A conforming implementation of this Reference Manual 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 Reference Manual for the semantics of the given program.

15.a
ramification
There is no need to produce any of the “internal effects” defined for the semantics of the program — all of these can be optimized away — so long as an appropriate sequence of external interactions is produced.
15.b
discussion
See also 11.6 which specifies various liberties associated with optimizations in the presence of language-defined checks, that could change the external effects that might be produced. These alternative external effects are still consistent with the standard, since 11.6 is part of the standard.
15.c
note
Note also that we only require “an appropriate sequence of external interactions” rather than “the same sequence...” An optimizer may cause a different sequence of external interactions to be produced than would be produced without the optimizer, so long as the new sequence still satisfies the requirements of the standard. For example, optimization might affect the relative rate of progress of two concurrent tasks, thereby altering the order in which two external interactions occur.
15.d/2
note
Note that the Ada 83 RM explicitly mentions the case of an “exact effect” of a program, but since so few programs have their effects defined that exactly, we don't even mention this “special” case. In particular, almost any program that uses floating point or tasking has to have some level of inexactness in the specification of its effects. And if one includes aspects of the timing of the external interactions in the external effect of the program (as is appropriate for a real-time language), no “exact effect” can be specified. For example, if two external interactions initiated by a single task are separated by a “delay 1.0;” then the language rules imply that the two external interactions have to be separated in time by at least one second, as defined by the clock associated with the delay_relative_statement. This in turn implies that the time at which an external interaction occurs is part of the characterization of the external interaction, at least in some cases, again making the specification of the required “exact effect” impractical.
16/5

An implementation that conforms to this Reference Manual shall support each capability required by the core language as specified. In addition, an implementation that conforms to this Reference Manual 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 shall be provided as specified.

16.a
discussion
The last sentence defines what it means to say that an implementation conforms to a Specialized Needs Annex, namely, only by supporting all capabilities required by the Annex.
17/3

An implementation conforming to this Reference Manual may provide additional aspects, attributes, library units, and pragmas. However, it shall not provide any aspect, attribute, library unit, or pragma having the same name as an aspect, 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.

17.a
discussion
The last sentence of the preceding paragraph defines what an implementation is allowed to do when it does not "conform" to a Specialized Needs Annex. In particular, the sentence forbids implementations from providing a construct with the same name as a corresponding construct in a Specialized Needs Annex but with a different syntax (e.g., an extended syntax) or quite different semantics. The phrase concerning "more limited in capability" is intended to give permission to provide a partial implementation, such as not implementing a subprogram in a package or having a restriction not permitted by an implementation that conforms to the Annex. For example, a partial implementation of the package Ada.Decimal might have Decimal.Max_Decimal_Digits as 15 (rather than the required 18). This allows a partial implementation to grow to a fully conforming implementation.
17.b
note
A restricted implementation might be restricted by not providing some subprograms specified in one of the packages defined by an Annex. In this case, a program that tries to use the missing subprogram will usually fail to compile. Alternatively, the implementation might declare the subprogram as abstract, so it cannot be called. Alternatively, a subprogram body might be implemented just to raise Program_Error. The advantage of this approach is that a program to be run under a fully conforming Annex implementation can be checked syntactically and semantically under an implementation that only partially supports the Annex. Finally, an implementation might provide a package declaration without the corresponding body, so that programs can be compiled, but partitions cannot be built and executed.
17.c
note
To ensure against wrong answers being delivered by a partial implementation, implementers are required to raise an exception when a program attempts to use an unsupported capability and this can be detected only at run time. For example, a partial implementation of Ada.Decimal might require the length of the Currency string to be 1, and hence, an exception would be raised if a subprogram were called in the package Edited_Output with a length greater than 1.
17.1/5

For an implementation that conforms to this Reference Manual, the implementation of a language-defined unit shall abide by all postconditions, type invariants, and default initial conditions specified for the unit by this document (see 11.4.2).

Documentation Requirements​

18

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 M.2.

18.a
discussion
We used to use the term “implementation dependent” instead of “unspecified”. However, that sounded too much like “implementation defined”. Furthermore, the term “unspecified” is used in the ANSI C and POSIX standards for this purpose, so that is another advantage. We also use “not specified” and “not specified by the language” as synonyms for “unspecified”. The documentation requirement is the only difference between implementation defined and unspecified.
18.b
note
Note that the “set of possible effects” can be “all imaginable effects”, as is the case with erroneous execution.
19

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.

19.a
discussion
For example, if the Reference Manual says that library unit elaboration order is implementation defined, the implementation might describe (in its user's manual) the algorithm it uses to determine the elaboration order. On the other hand, the implementation might provide a command that produces a description of the elaboration order for a partition upon request from the user. It is also acceptable to provide cross references to existing documentation (for example, a hardware manual), where appropriate.
19.b
note
Note that dependence of a program on implementation-defined or unspecified functionality is not defined to be an error; it might cause the program to be less portable, however.
19.c/2
note
Documentation Requirement: The behavior of implementations in implementation-defined situations shall be documented — see M.2, “Implementation-Defined Characteristics” for a listing.

Implementation Advice​

20

If an implementation detects the use of an unsupported Specialized Needs Annex feature at run time, it should raise Program_Error if feasible.

20.a.1/2
implementation advice
Program_Error should be raised when an unsupported Specialized Needs Annex feature is used at run time.
20.a
reason
The reason we don't require Program_Error is that there are situations where other exceptions might make sense. For example, if the Real Time Systems Annex requires that the range of System.Priority include at least 30 values, an implementation could conform to the Reference Manual (but not to the Annex) if it supported only 12 values. Since the rules of the language require Constraint_Error to be raised for out-of-range values, we cannot require Program_Error to be raised instead.
21

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.

21.a.1/2
implementation advice
Implementation-defined extensions to the functionality of a language-defined library unit should be provided by adding children to the library unit.
21.a
implementation note
If an implementation has support code (“run-time system code”) that is needed for the execution of user-defined code, it can put that support code in child packages of System. Otherwise, it has to use some trick to avoid polluting the user's namespace. It is important that such tricks not be available to user-defined code (not in the standard mode, at least) — that would defeat the purpose.
22/5
note
NOTE The above requirements imply that an implementation conforming to this Reference Manual can support some of the capabilities required by a Specialized Needs Annex without supporting all required capabilities.
22.a
discussion
A conforming implementation can partially support a Specialized Needs Annex. Such an implementation does not conform to the Annex, but it does conform to the Reference Manual.

1.1.4 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:

4
  • Lower case words in a sans-serif font, some containing embedded underlines, are used to denote syntactic categories, for example:
5

case_statement

6
  • Boldface words are used to denote reserved words, for example:
7

array

8
  • Square brackets enclose optional items. Thus the two following rules are equivalent.
9/2
simple_return_statement ::= return [expression];
simple_return_statement ::= return; | return expression;
10
  • 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.
11
term ::= factor {multiplying_operator factor}
term ::= factor | term multiplying_operator factor
12/5
  • A vertical line separates alternative items, for example :
13/5
constraint ::= scalar_constraint | composite_constraint
13.1/5
  • For symbols used in this notation (square brackets, curly brackets, and the vertical line), the symbols when surrounded by ' represent themselves, for example:
13.2/5
discrete_choice_list ::= discrete_choice {'|' discrete_choice}
named_container_aggregate ::= '[' container_element_association_list ']'
14
  • 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.
14.a
discussion
The grammar given in this document is not LR(1). In fact, it is ambiguous; the ambiguities are resolved by the overload resolution rules (see 8.6).
14.b
note
We often use “if” to mean “if and only if” in definitions. For example, if we define “photogenic” by saying, “A type is photogenic if it has the following properties...”, we mean that a type is photogenic if and only if it has those properties. It is usually clear from the context, and adding the “and only if” seems too cumbersome.
14.c
note
When we say, for example, “a declarative_item of a declarative_part”, we are talking about a declarative_item immediately within that declarative_part. When we say “a declarative_item in, or within, a declarative_part”, we are talking about a declarative_item anywhere in the declarative_part, possibly deeply nested within other declarative_parts. (This notation doesn't work very well for names, since the name “of” something also has another meaning.)
14.d
note
When we refer to the name of a language-defined entity (for example, Duration), we mean the language-defined entity even in programs where the declaration of the language-defined entity is hidden by another declaration. For example, when we say that the expected type for the expression of a delay_relative_statement is Duration, we mean the language-defined type Duration that is declared in Standard, not some type Duration the user might have declared.
14.1/3

The delimiters, compound delimiters, reserved words, and numeric_literals are exclusively made of the characters whose code point is between 16#20# and 16#7E#, inclusively. The special characters for which names are defined in this document (see 2.1) belong to the same range. [For example, the character E in the definition of exponent is the character whose name is “LATIN CAPITAL LETTER E”, not “GREEK CAPITAL LETTER EPSILON”.]

14.e/2
discussion
This just means that programs can be written in plain ASCII characters; no characters outside of the 7-bit range are required.
14.2/5

When this document mentions the conversion of some character or sequence of characters to upper case, it means the character or sequence of characters obtained by using simple upper case mapping, as defined by documents referenced in Clause 2 of ISO/IEC 10646:2017 .

14.f/5
implementation note
The “documents referenced” means Unicode, Chapter 4 (specifically, section 4.2 — Case). Machine-readable versions of Simple Uppercase Mapping and Simple Lowercase Mapping can be found in http://www.unicode.org/Public/UCD/latest/ucd/UnicodeData.txt. Data for older Unicode versions can be found on this site as well; start at http://www.unicode.org/Public/ and find the appropriate version number. Simple Uppercase Mapping is the 12th field in this file (the 13th element of each line, since Unicode counts from 0); the Simple Lowercase Mapping is the 13th field in this file. In both cases, if no character is present in the field, the character maps to itself.
15

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.

16

A construct is a piece of text (explicit or implicit) that is an instance of a syntactic category defined under “Syntax”. Version=[5],Kind=(AddedNormal),Group=[C],Term=[construct], Def=[a piece of text (explicit or implicit) that is an instance of a syntactic category defined under Syntax]

16.a
ramification
For example, an expression is a construct. A declaration is a construct, whereas the thing declared by a declaration is an “entity”.
16.b
discussion
“Explicit” and “implicit” don't mean exactly what you might think they mean: The text of an instance of a generic is considered explicit, even though it does not appear explicitly (in the nontechnical sense) in the program text, and even though its meaning is not defined entirely in terms of that text.
17

A constituent of a construct is the construct itself, or any construct appearing within it.

18

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 runtime 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.]

18.a
discussion
Programs will be more portable if their external effect does not depend on the particular order chosen by an implementation.
18.b
ramification
Additional reordering permissions are given in 11.6, “Exceptions and Optimization”.
18.c
note
There is no requirement that the implementation always choose the same order in a given kind of situation. In fact, the implementation is allowed to choose a different order for two different executions of the same construct. However, we expect most implementations will behave in a relatively predictable manner in most situations.
18.d
reason
The “sequential order” wording is intended to allow the programmer to rely on “benign” side effects. For example, if F is a function that returns a unique integer by incrementing some global and returning the result, a call such as P(F, F) is OK if the programmer cares only that the two results of F are unique; the two calls of F cannot be executed in parallel, unless the compiler can prove that parallel execution is equivalent to some sequential order.
19
note
NOTE 1 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:
20
if_statement ::=
    if condition then
      sequence_of_statements
   {elsif condition then
      sequence_of_statements}
   [else
      sequence_of_statements]
    end if;
21
note
NOTE 2 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.

Wording Changes from Ada 95​

21.a/2
note
We now explicitly say that the lexical elements of the language (with a few exceptions) are made up of characters in the lower half of the Latin-1 character set. This is needed to avoid confusion given the new capability to use most ISO 10646 characters in identifiers and strings.
21.b/2
note
We now explicitly define what the Reference Manual means by upper case, as there are many possibilities for ISO 10646 characters.
21.c/2
note
The example for square brackets has been changed as there is no longer a return_statement syntax rule.

Wording Changes from Ada 2005​

21.d/3
correction
Upper case is defined by "simple upper case mapping", because "full case folding" is a mapping (mostly) to lower case.

Wording Changes from Ada 2012​

21.e/5
note
Changed the syntax description to quote symbols used in the notation when they appear as themselves. We made this change to allow square brackets in Ada syntax, but the change also allowed us to get rid of an old hack which allowed vertical lines to stand for themselves in limited circumstances.

1.1.5 Classification of Errors​

Language Design Principles​

0.a/5
note
When the Reference Manual says that some construct C1 has equivalent dynamic semantics to some other construct C2, then there should be a language rule that says that C1 is illegal if C2 is illegal.
0.b/5
reason
We do not want to infer Legality Rules from Dynamic Semantics rules.
0.c/5
note
When the value of an expression is needed to enforce a Legality Rule or Post-Compilation Rule, then there should be a language rule requiring the expression to be static (see 4.9).
0.d/5
discussion
This most likely occurs when the expression specifies the value of an aspect.

Implementation Requirements​

1

The language definition classifies errors into several different categories:

2
  • Errors that are required to be detected prior to run time by every Ada implementation;
    • 3
  • These errors correspond to any violation of a rule given in this document, 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.
    • 4
  • 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.
4.a
ramification
See, for example, 10.1.3, “Subunits of Compilation Units”, for some errors that are detected only after compilation. Implementations are allowed, but not required, to detect post compilation rules at compile time when possible.
5
  • Errors that are required to be detected at run time by the execution of an Ada program;
    • 6
  • 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.]
    • 7
  • Bounded errors;
    • 8/5
  • The language rules define certain kinds of errors that are not expected to 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.
    • 9
  • Erroneous execution.
    • 10/5
  • In addition to bounded errors, the language rules define certain kinds of errors as leading to erroneous execution. Like bounded errors, the implementation is not expected to 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.
10.a
ramification
Executions are erroneous, not programs or parts of programs. Once something erroneous happens, the execution of the entire program is erroneous from that point on, and potentially before given possible reorderings permitted by 11.6 and elsewhere. We cannot limit it to just one partition, since partitions are not required to live in separate address spaces. (But implementations are encouraged to limit it as much as possible.)
10.b
note
Suppose a program contains a pair of things that will be executed “in an arbitrary order”. It is possible that one order will result in something sensible, whereas the other order will result in erroneous execution. If the implementation happens to choose the first order, then the execution is not erroneous. This may seem odd, but it is not harmful.
10.c
note
Saying that something is erroneous is semantically equivalent to saying that the behavior is unspecified. However, “erroneous” has a slightly more disapproving flavor.

Implementation Permissions​

11

[ 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 Reference Manual; in particular, in the standard mode, all legal compilation_units shall be accepted.]

11.a
discussion
These permissions are designed to authorize explicitly the support for alternative modes. Of course, nothing we say can prevent them anyway, but this (redundant) paragraph is designed to indicate that such alternative modes are in some sense “approved” and even encouraged where they serve the specialized needs of a given user community, so long as the standard mode, designed to foster maximum portability, is always available.

Implementation Advice​

12

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

12.a.1/2
implementation advice
If a bounded error or erroneous execution is detected, Program_Error should be raised.

Wording Changes from Ada 83​

12.a
note
Some situations that are erroneous in Ada 83 are no longer errors at all. For example, depending on the parameter passing mechanism when unspecified is possibly nonportable, but not erroneous.
12.b
note
Other situations that are erroneous in Ada 83 are changed to be bounded errors. In particular, evaluating an uninitialized scalar variable is a bounded error. The possible results are to raise Program_Error (as always), or to produce a machine-representable value (which might not be in the subtype of the variable). Violating a Range_Check or Overflow_Check raises Constraint_Error, even if the value came from an uninitialized variable. This means that optimizers can no longer “assume” that all variables are initialized within their subtype's range. Violating a check that is suppressed remains erroneous.
12.c
note
The “incorrect order dependences” category of errors is removed. All such situations are simply considered potential nonportabilities. This category was removed due to the difficulty of defining what it means for two executions to have a “different effect”. For example, if a function with a side effect is called twice in a single expression, it is not in principle possible for the compiler to decide whether the correctness of the resulting program depends on the order of execution of the two function calls. A compile time warning might be appropriate, but raising of Program_Error at run time would not be.