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6.1 Subprogram Declarations

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A subprogram_declaration declares a procedure or function. 

Syntax

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subprogram_declaration ::= 
    [overriding_indicator]
    subprogram_specification;
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This paragraph was deleted.
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subprogram_specification ::= 
    procedure_specification
  | function_specification
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procedure_specification ::= procedure defining_program_unit_name parameter_profile
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function_specification ::= function defining_designator parameter_and_result_profile
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designator ::= [parent_unit_name . ]identifier | operator_symbol
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defining_designator ::= defining_program_unit_name | defining_operator_symbol
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defining_program_unit_name ::= [parent_unit_name . ]defining_identifier
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The optional parent_unit_name is only allowed for library units (see 10.1.1).
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operator_symbol ::= string_literal
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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 clause 4.5.
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defining_operator_symbol ::= operator_symbol
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parameter_profile ::= [formal_part]
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parameter_and_result_profile ::= 
    [formal_partreturn [null_exclusionsubtype_mark
  | [formal_partreturn access_definition
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formal_part ::= 
   (parameter_specification {; parameter_specification})
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parameter_specification ::= 
    defining_identifier_list : mode [null_exclusionsubtype_mark [:= default_expression]
  | defining_identifier_list : access_definition [:= default_expression]
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mode ::= [in] | in out | out

Name Resolution Rules

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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

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The parameter mode of a formal parameter conveys the direction of information transfer with the actual parameter: in, in out, or out. Mode in is the default, and is the mode of a parameter defined by an access_definition. The formal parameters of a function, if any, shall have the mode in.
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A default_expression is only allowed in a parameter_specification for a formal parameter of mode in.
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 A subprogram_declaration or a generic_subprogram_declaration requires a completion: a body, a renaming_declaration (see 8.5), or a pragma Import (see B.1). A completion is not allowed for an abstract_subprogram_declaration (see 3.9.3) or a null_procedure_declaration (see 6.7). 
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A name that denotes a formal parameter is not allowed within the formal_part in which it is declared, nor within the formal_part of a corresponding body or accept_statement.

Static Semantics

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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.
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 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.
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 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.
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The subtypes of a profile are: 
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The types of a profile are the types of those subtypes.
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 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 defined by a null_procedure_declaration is a null procedure; a procedure declared by a subprogram_declaration is not. See 6.7, “Null Procedures”.
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   An overriding_indicator is used to indicate whether overriding is intended. See 8.3.1, “Overriding Indicators”. 

Dynamic Semantics

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 The elaboration of a subprogram_declaration has no effect. 
NOTES
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1  A parameter_specification with several identifiers is equivalent to a sequence of single parameter_specifications, as explained in 3.3.
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2  Abstract subprograms do not have bodies, and cannot be used in a nondispatching call (see 3.9.3, “Abstract Types and Subprograms”).
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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.
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4  Subprograms can be called recursively and can be called concurrently from multiple tasks. 

Examples

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Examples of subprogram declarations: 
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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
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function Random return Probability;                      --  see 3.5.7
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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
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function "*"(Left, Right : Matrix) return Matrix;        --  see 3.6
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Examples of in parameters with default expressions: 
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procedure Print_Header(Pages  : in Natural;
            Header : in Line    :=  (1 .. Line'Last => ' ');  --  see 3.6
            Center : in Boolean := True);

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