Testing passing function

I have callback function and want to make sure that Func only called when it is not nil and make it default to nil.

type
  TCallBack = procedure of object

procedure TBlabla.Loop(Func: TCallBack = nil);
begin
  if Func <> nil then Func;
end;
comicboyAsked:
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mikepjConnect With a Mentor Commented:
Hi ComicBoy,

The full answer is below.  I have provided the answer you need.  Using a pointer to a procedure is a rather special possibility which I use only rarely (mostly for sorting routines to provide a function which performs the evaluation to decide whether item x should go before or after item y.  A pointer points at something.  Usually a pointer points at a variable but it could point at other things (like a procedure).

Since it sounds like you're a beginner, I would suggest that you think of other ways you might do what you have asked.  It's just that it's a bit advanced and can possibly cause you some frustration if you don't fully understand all the concepts involved.

If you need further help with this, ask away.  It'd be good if you are using pointers and such for you to do some reading about them.  Have fun; very powerful stuff!

MP

unit Unit1;

interface

uses
  Windows, Messages, SysUtils, Classes, Graphics, Controls, Forms, Dialogs;

type
  TCallBack = procedure;

  TForm1 = class(TForm)
    procedure FormCreate(Sender: TObject);
  private
    { Private declarations }
  public
    { Public declarations }
    procedure Loop(Func: TCallBack = nil);
  end;

var
  Form1: TForm1;

procedure f;

implementation

{$R *.DFM}

procedure f;
begin
  Showmessage('hi');
end;

procedure TForm1.Loop(Func: TCallBack = nil);
begin
 if (@Func <> nil) then Func;
end;

procedure TForm1.FormCreate(Sender: TObject);
begin
  // let's test it
  Loop(nil);
  Loop(@f);
end;

end.
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mikepjCommented:
"procedure of object" -> "procedure"
"func <> nil" -> "@func<>nil"

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nestoruaCommented:
HI,
What about: if Assigned(Func) then Func.
Sincerely,
Nestorua.
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mikepjCommented:
Assigned() is the same as <>nil
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nestoruaCommented:
But more DELPHIable and not the same when TFunc has
parameters, if I am not mistaken.
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mikepjCommented:
Myself I prefer to use Assigned().  From the Delphi help file:

"Assigned(P) corresponds to the test P<> nil for a pointer variable, and @P <> nil for a procedural variable."
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nestoruaCommented:
That's what I tried to say. Agree with you, mikepj.
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comicboyAuthor Commented:
Hi, I'm newbie in Delphi could you please tell me what example of a "ponter variable" and what is "procedural variable" actually in program ?

Anyway both of you expert in this field who should I give the points ?
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comicboyAuthor Commented:
So sorry for long delay on grading, I'm away awhile from internet reach...

Anyway Thanks alot for your great answer of massive knowledge :) you are a great expert :)
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