Annotated Ada Reference Manual — Legal Information

6.7 Null Procedures

1/2

{AI95-00348-01} A null_procedure_declaration provides a shorthand to declare a procedure with an empty body.

Syntax

2/2

{AI95-00348-01} null_procedure_declaration ::=
[overriding_indicator]
procedure_specification is null;

Static Semantics

3/2

{AI95-00348-01} A null_procedure_declaration declares a null procedure.{null procedure} {procedure (null)} A completion is not allowed for a null_procedure_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

4/2

{AI95-00348-01} The execution of a null procedure is invoked by a subprogram call. For the execution of a subprogram call on a null procedure, 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.

5/2

{AI95-00348-01} {elaboration (null_procedure_declaration) [partial]} The elaboration of a null_procedure_declaration has no effect.

Examples

6/2

{AI95-00433-01} procedure Simplify(Expr : in out Expression) is null; -- see 3.9
-- By default, Simplify does nothing, but it may be overridden in extensions of Expression

Extensions to Ada 95

6.a/2

{AI95-00348-01} {extensions to Ada 95} Null procedures are new.