Article #15447: Using procedural varialbles in Pascal

 Technical Information Database

TI447D.txt - Using procedural varialbles in Pascal

Category   :Object Pascal
Platform   :All-32Bit
Product    :

Description:

  The included program, FUNC1.PAS shows how to use procedural
  variables.

  It creates both a pointer to a function and a pointer
  to a procedure and shows how to use them.

  Func1 creates a pointer to a function called MyFunc,
  calls it once and then passes it to a procedure which
  then proceeds to call it. Therefore this examples shows
  how to assign and call a procedural variable, and
  also how to pass it to another procedure which
  can then call it.

  The following lines demonstrate how to create types
  that can point at a procedure or function:

  type
    TMyFunc = function(i: Integer): Integer;
    TMyProc = procedure;

  Notice that the actual name of the function or procedure
  is left out, but that it is necessary to declare any
  variables passed to function or procedure, and to specify
  any return types from a function.

  Its important to understand that TMyFunc and TMyProc are
  really pointers to functions, even though there is no
  need to call new and dispose, and no need to dereference
  the pointer with the caret symbol ("^").

  Notice also that both MyProcedure and MyFunc are declared
  "Far". This is absolutely necessary when working with
  procedural variables, though of course functions declared
  in the header to a unit will automatically be far when
  called from another module.


program Func1;

type
  TMyFunc = function(i: Integer): Integer;
  TMyProc = procedure;

procedure MyProcedure; far;
begin
  WriteLn('Hi');
end;

function MyFunc(I: Integer): Integer; far;
begin
  WriteLn(i);
end;

procedure Foo(F: TMyFunc);
begin
  F(5);
end;

var
  F: TMyFunc;
  P: TMyProc;
begin
  F := MyFunc;
  F(10);
  Foo(F);
  P := MyProcedure;
  P;
end.


Reference:
 

4/22/99 1:02:30 PM
 

Last Modified: 01-SEP-99