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6.8 Expression Functions

An expression_function_declaration provides a shorthand to declare a function whose body consists of a single return statement. 


expression_function_declaration ::= 
   function_specification is
 | [overriding_indicator]
   function_specification is

Name Resolution Rules

The expected type for the expression or aggregate of an expression_function_declaration is the result type (see 6.5) of the function.

Static Semantics

 An expression_function_declaration that is not a completion declares an expression function. The return expression of an expression function is the expression or aggregate of the expression_function_declaration. A completion is not allowed for an expression_function_declaration; however, an expression_function_declaration can complete a previous declaration.
 A potentially static expression is defined in the same way as a static expression except that
a name denoting a formal parameter of an expression function is a potentially static expression; and
each use of “static expression” in the definition of “static expression” is replaced with a corresponding use of “potentially static expression” in the definition of “potentially static expression”. 
 The following language-defined representation aspect may be specified for an expression function:
The type of aspect Static is Boolean. When aspect Static is True for an expression function, the function is a static expression function. If directly specified, the aspect_definition shall be a static expression.
The Static value for an inherited function is True if some corresponding primitive function of the parent or progenitor type is a static expression function; otherwise, if not directly specified, the aspect is False.
 A static expression function is a static function; see 4.9

Legality Rules

If an expression_function_declaration is a completion, it shall be the completion of a subprogram_declaration or generic_subprogram_declaration. The profile of an expression_function_declaration that completes a declaration shall conform fully to that of the declaration.
If the result subtype has one or more unconstrained access discriminants, the accessibility level of the anonymous access type of each access discriminant, as determined by the expression or aggregate of the expression_function_declaration, shall not be statically deeper than that of the master that elaborated the expression_function_declaration.
 Aspect Static shall be specified to have the value True only if the associated expression_function_declaration:
is not a completion;
has an expression that is a potentially static expression;
contains no calls to itself;
each parameter (if any) is of mode in and is of a static subtype;
has a result subtype that is a static subtype;
has no applicable precondition or postcondition expression; and
for result type R, if the function is a boundary entity for type R (see 7.3.2), no type invariant applies to type R; if R has a component type C, a similar rule applies to C
Paragraph 6 was deleted.

Dynamic Semantics

The execution of an expression function is invoked by a subprogram call. For the execution of a subprogram call on an expression function, or on a function completed with a expression_function_declaration, the execution of the subprogram_body executes an implicit function body containing only a simple_return_statement whose expression is the return expression of the expression function.
The elaboration of an expression_function_declaration has no other effect than to establish that the expression function can be called without failing the Elaboration_Check. 


Example of an expression function: 
function Is_Origin (P : in Point) return Boolean is -- see 3.9
   (P.X = 0.0 and P.Y = 0.0);

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