Ada Reference Manual (Ada 2022)
6.7 Null Procedures
A null_procedure_declaration provides a shorthand to declare a procedure with an empty body.
Syntax
null_procedure_declaration ::=
[overriding_indicator]
procedure_specification is null
[aspect_specification];
[overriding_indicator]
procedure_specification is null
[aspect_specification];
Legality Rules
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
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.
Dynamic Semantics
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.
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
Example of the declaration of a null procedure:
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
-- By default, Simplify does nothing, but it can be overridden in extensions of Expression
