6.7. Null Procedures

danger

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1/2

A null_procedure_declaration provides a shorthand to declare a procedure with an empty body.

Syntax

2/3

null_procedure_declaration ::= 
   [overriding_indicator]
   procedure_specification is null
       [aspect_specification];

Legality Rules

2.1/3

If a null_procedure_declaration is a completion, it shall be the completion of a subprogram_declaration or generic_subprogram_declaration. The profile of a null_procedure_declaration that completes a declaration shall conform fully to that of the declaration.

Static Semantics

3/5

A null_procedure_declaration that is not a completion declares a null procedure. A completion is not allowed for a null_procedure_declaration; however, a null_procedure_declaration can complete a previous declaration.

3.a/2

reason

There are no null functions because the return value has to be constructed somehow; a function that always raises Program_Error doesn't seem very useful or worth the complication.

Dynamic Semantics

4/5

The execution of a null procedure is invoked by a subprogram call. For the execution of a subprogram call on a null procedure, or on a procedure completed with a null_procedure_declaration, the execution of the subprogram_body has no effect.

4.a/2

ramification

Thus, a null procedure is equivalent to the body

4.b/2

begin
   null;
end;

4.c/2

with the exception that a null procedure can be used in place of a procedure specification.

5/3

The elaboration of a null_procedure_declaration has no other effect than to establish that the null procedure can be called without failing the Elaboration_Check.

Examples

6/5

Example of the declaration of a null procedure:

7/5

procedure Simplify(Expr : in out Expression) is null; -- see 3.9
-- By default, Simplify does nothing, but it can be overridden in extensions of Expression

Extensions to Ada 95

7.a/2

Null procedures are new.

Extensions to Ada 2005

7.b/3

A null_procedure_declaration can now be a completion.

7.c/3

An optional aspect_specification can be used in a null_procedure_declaration. This is described in 13.1.1.

Wording Changes from Ada 2012

7.d/5

Clarified the term “null procedure” so it matches the meaning expected in 6.1.