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6.1 Subprogram Declarations

1
[A subprogram_declaration declares a procedure or function.] 

Syntax

2/3
{AI95-00218-03} {AI05-0183-1} subprogram_declaration ::= 
    [overriding_indicator]
    subprogram_specification
        [aspect_specification];
3/2
This paragraph was deleted.{AI95-00348-01}
4/2
{AI95-00348-01} subprogram_specification ::= 
    procedure_specification
  | function_specification
4.1/2
{AI95-00348-01} procedure_specification ::= procedure defining_program_unit_name parameter_profile
4.2/2
{AI95-00348-01} function_specification ::= function defining_designator parameter_and_result_profile
5
designator ::= [parent_unit_name . ]identifier | operator_symbol
6
defining_designator ::= defining_program_unit_name | defining_operator_symbol
7
defining_program_unit_name ::= [parent_unit_name . ]defining_identifier
8
[The optional parent_unit_name is only allowed for library units (see 10.1.1).]
9
operator_symbol ::= string_literal
10/3
{AI95-00395-01} {AI05-0299-1} The sequence of characters in an operator_symbol shall form a reserved word, a delimiter, or compound delimiter that corresponds to an operator belonging to one of the six categories of operators defined in subclause 4.5.
10.a/3
Reason: {AI95-00395-01} {AI05-0090-1} The “sequence of characters” of the string literal of the operator is a technical term (see 2.6), and does not include the surrounding quote characters. As defined in 2.2, lexical elements are “formed” from a sequence of characters. Spaces are not allowed, and upper and lower case is not significant.
11
defining_operator_symbol ::= operator_symbol
12
parameter_profile ::= [formal_part]
13/2
{AI95-00231-01} {AI95-00318-02} parameter_and_result_profile ::= 
    [formal_partreturn [null_exclusionsubtype_mark
  | [formal_partreturn access_definition
14
formal_part ::= 
   (parameter_specification {; parameter_specification})
15/3
{AI95-00231-01} {AI05-0142-4} parameter_specification ::= 
    defining_identifier_list : [aliasedmode [null_exclusionsubtype_mark [:= default_expression]
  | defining_identifier_list : access_definition [:= default_expression]
16
mode ::= [in] | in out | out

Name Resolution Rules

17
A formal parameter is an object [directly visible within a subprogram_body] that represents the actual parameter passed to the subprogram in a call; it is declared by a parameter_specification. For a formal parameter, the expected type for its default_expression, if any, is that of the formal parameter.

Legality Rules

18/3
{AI05-0143-1} The parameter mode of a formal parameter conveys the direction of information transfer with the actual parameter: in, in out, or out. Mode in is the default, and is the mode of a parameter defined by an access_definition.
18.a/3
This paragraph was deleted.{AI05-0143-1}
19
A default_expression is only allowed in a parameter_specification for a formal parameter of mode in.
20/3
{AI95-00348-01} {AI05-0177-1} {AI05-0229-1} A subprogram_declaration or a generic_subprogram_declaration requires a completion [unless the Import aspect (see B.1) is True for the declaration; the completion shall be a body or a renaming_declaration (see 8.5)]. [A completion is not allowed for an abstract_subprogram_declaration (see 3.9.3), a null_procedure_declaration (see 6.7), or an expression_function_declaration (see 6.8).] 
20.a/3
Ramification: {AI95-00348-01} {AI05-0177-1} Abstract subprograms , null procedures, and expression functions are not declared by subprogram_declarations, and so do not require completion (although the latter two can be completions). Protected subprograms are declared by subprogram_declarations, and so require completion. Note that an abstract subprogram is a subprogram, a null procedure is a subprogram, an expression function is a subprogram, and a protected subprogram is a subprogram, but a generic subprogram is not a subprogram. 
20.b/3
Proof: {AI05-0229-1} When the Import aspect is True for any entity, no completion is allowed (see B.1). 
21
A name that denotes a formal parameter is not allowed within the formal_part in which it is declared, nor within the formal_part of a corresponding body or accept_statement.
21.a
Ramification: By contrast, generic_formal_parameter_declarations are visible to subsequent declarations in the same generic_formal_part.

Static Semantics

22
The profile of (a view of) a callable entity is either a parameter_profile or parameter_and_result_profile[; it embodies information about the interface to that entity — for example, the profile includes information about parameters passed to the callable entity. All callable entities have a profile — enumeration literals, other subprograms, and entries. An access-to-subprogram type has a designated profile.] Associated with a profile is a calling convention. A subprogram_declaration declares a procedure or a function, as indicated by the initial reserved word, with name and profile as given by its specification.
23/2
{AI95-00231-01} {AI95-00318-02} The nominal subtype of a formal parameter is the subtype determined by the optional null_exclusion and the subtype_mark, or defined by the access_definition, in the parameter_specification. The nominal subtype of a function result is the subtype determined by the optional null_exclusion and the subtype_mark, or defined by the access_definition, in the parameter_and_result_profile.
23.1/3
  {AI05-0142-4} An explicitly aliased parameter is a formal parameter whose parameter_specification includes the reserved word aliased.
24/2
{AI95-00231-01} {AI95-00254-01} {AI95-00318-02} An access parameter is a formal in parameter specified by an access_definition. An access result type is a function result type specified by an access_definition. An access parameter or result type is of an anonymous access type (see 3.10). [Access parameters of an access-to-object type allow dispatching calls to be controlled by access values. Access parameters of an access-to-subprogram type permit calls to subprograms passed as parameters irrespective of their accessibility level.]
24.a/2
Discussion: {AI95-00318-02} Access result types have normal accessibility and thus don't have any special properties worth noting here. 
25
The subtypes of a profile are: 
26
For any non-access parameters, the nominal subtype of the parameter.
27/2
{AI95-00254-01} For any access parameters of an access-to-object type, the designated subtype of the parameter type.
27.1/3
{AI95-00254-01} {AI05-0164-1} For any access parameters of an access-to-subprogram type, the subtypes of the designated profile of the parameter type.
28/2
{AI95-00231-01} {AI95-00318-02} For any non-access result, the nominal subtype of the function result.
28.1/2
{AI95-00318-02} For any access result type of an access-to-object type, the designated subtype of the result type.
28.2/3
{AI95-00318-02} {AI05-0164-1} For any access result type of an access-to-subprogram type, the subtypes of the designated profile of the result type.
29
[ The types of a profile are the types of those subtypes.]
30/3
{AI95-00348-01} {AI05-0177-1} [A subprogram declared by an abstract_subprogram_declaration is abstract; a subprogram declared by a subprogram_declaration is not. See 3.9.3, “Abstract Types and Subprograms”. Similarly, a procedure declared by a null_procedure_declaration is a null procedure; a procedure declared by a subprogram_declaration is not. See 6.7, “Null Procedures”. Finally, a function declared by an expression_function_declaration is an expression function; a function declared by a subprogram_declaration is not. See 6.8, “Expression Functions”.]
30.1/2
  {AI95-00218-03} [An overriding_indicator is used to indicate whether overriding is intended. See 8.3.1, “Overriding Indicators”.] 

Dynamic Semantics

31/2
{AI95-00348-01} The elaboration of a subprogram_declaration has no effect. 
NOTES
32
1  A parameter_specification with several identifiers is equivalent to a sequence of single parameter_specifications, as explained in 3.3.
33
2  Abstract subprograms do not have bodies, and cannot be used in a nondispatching call (see 3.9.3, “Abstract Types and Subprograms”).
34
3  The evaluation of default_expressions is caused by certain calls, as described in 6.4.1. They are not evaluated during the elaboration of the subprogram declaration.
35
4  Subprograms can be called recursively and can be called concurrently from multiple tasks. 

Examples

36
Examples of subprogram declarations: 
37
procedure Traverse_Tree;
procedure Increment(X : in out Integer);
procedure Right_Indent(Margin : out Line_Size);          --  see 3.5.4
procedure Switch(From, To : in out Link);                --  see 3.10.1
38
function Random return Probability;                      --  see 3.5.7
39/4
{AI12-0056-1} function Min_Cell(X : Link) return Cell;                 --  see 3.10.1
function Next_Frame(K : Positive) return Frame;          --  see 3.10
function Dot_Product(Left, Right : Vector) return Real;  --  see 3.6
function Find(B : aliased in out Barrel; Key : String) return Real;
                                                         --  see 4.1.5
40
function "*"(Left, Right : Matrix) return Matrix;        --  see 3.6
41
Examples of in parameters with default expressions: 
42
procedure Print_Header(Pages  : in Natural;
            Header : in Line    :=  (1 .. Line'Last => ' ');  --  see 3.6
            Center : in Boolean := True);

Extensions to Ada 83

42.a
The syntax for abstract_subprogram_declaration is added. The syntax for parameter_specification is revised to allow for access parameters (see 3.10)
42.b/3
{AI05-0299-1} Program units that are library units may have a parent_unit_name to indicate the parent of a child (see 10.1.1).

Wording Changes from Ada 83

42.c
We have incorporated the rules from RM83-6.5, “Function Subprograms” here and in 6.3, “Subprogram Bodies
42.d
We have incorporated the definitions of RM83-6.6, “Parameter and Result Type Profile - Overloading of Subprograms” here.
42.e
The syntax rule for defining_operator_symbol is new. It is used for the defining occurrence of an operator_symbol, analogously to defining_identifier. Usage occurrences use the direct_name or selector_name syntactic categories. The syntax rules for defining_designator and defining_program_unit_name are new. 

Extensions to Ada 95

42.f/2
{AI95-00218-03} Subprograms now allow overriding_indicators for better error checking of overriding.
42.g/2
{AI95-00231-01} An optional null_exclusion can be used in a formal parameter declaration. Similarly, an optional null_exclusion can be used in a function result.
42.h/2
{AI95-00318-02} The return type of a function can be an anonymous access type. 

Wording Changes from Ada 95

42.i/2
{AI95-00254-01} A description of the purpose of anonymous access-to-subprogram parameters and the definition of the profile of subprograms containing them was added.
42.j/2
{AI95-00348-01} Split the production for subprogram_specification in order to make the declaration of null procedures (see 6.7) easier.
42.k/2
{AI95-00348-01} Moved the Syntax and Dynamic Semantics for abstract_subprogram_declaration to 3.9.3, so that the syntax and semantics are together. This also keeps abstract and null subprograms similar.
42.l/2
{AI95-00395-01} Revised to allow other_format characters in operator_symbols in the same way as the underlying constructs. 

Extensions to Ada 2005

42.m/3
{AI05-0142-4} Parameters can now be explicitly aliased, allowing parts of function results to designate parameters and forcing by-reference parameter passing.
42.n/3
{AI05-0143-1} The parameters of a function can now have any mode.
42.o/3
{AI05-0183-1} An optional aspect_specification can be used in a subprogram_declaration. This is described in 13.1.1

Wording Changes from Ada 2005

42.p/3
{AI05-0177-1} Added expression functions (see 6.8) to the wording. 

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