Question about assignment

[stab]

In a library written FPC there is a property like:
  TClipper = class(TClipperBase)
  private
    FZFillCallback    : TZFillCallback;
  public
    property ZFillFunction: TZFillCallback read FZFillCallback write
       SetZFillCallback;
  end;

procedure TClipper.SetZFillCallback(AValue: TZFillCallback);
begin
  if not Assigned(FZFillCallback) then
    FZFillCallback:=AValue;
end;

When accessing SetZFillCallback I see that FZFillCallback is not
assigned and thus FZFillCallback:=AValue; is executed.
Investigating the assignment before and after reveals that  the value of FZFillCallback is not changed. HOW COME???

[lucamar]

Shouldn't that be:

Code: Pascal  [Select][+][-]

1. TClipper = class(TClipperBase)
2. private
3.   FZFillCallback    : TZFillCallback;
4.   procedure SetZFillCallback(AValue: TZFillCallback);
5. public
6.   property ZFillFunction: TZFillCallback read FZFillCallback write
7.        SetZFillCallback;
8. end;

???

Otherwise, if the parent class TClipperBase (or any ancestor of it) has a SetZFillCallback(), it's that which will be executed (possibly setting other field or doing nothing at all)

[stab]

Sorry, I don't see any difference between my code and your suggestion. From a calling unit I see that procedure SetZFillCallback is entered but there isn't any change in the FZFillCallback procedure variable.

[lucamar]

Sorry, I don't see any difference between my code and your suggestion.

Your posted code lacks the declaration of the setter; if it's different in the real code then I don't know what may be happening, sorry.

Only one thing: If your code is in mode ObjFPC then maybe the setter should be like:

Code: Pascal  [Select][+][-]

1. procedure TClipper.SetZFillCallback(const AValue: TZFillCallback);
2. begin
3.   if not Assigned(FZFillCallback) then
4.     FZFillCallback := @AValue;
5. end;

???

Though I'm not entirely sure of that ...

Or maybe this?:

Code: Pascal  [Select][+][-]

1. procedure TClipper.SetZFillCallback(AValue: TZFillCallback);
2. begin
3.   if FZFillCallback <> AValue then
4.     FZFillCallback := AValue;
5. end;

[stab]

The problem is not that the code doesn't reach the assignment but that the assignment doesn't have any effect, i.e FZFillCallback is unaffected.

[jamie]

Did you create the instance first ?

Also, in the constructor set the current value to NIL so that Assign will Fail and thus you will actually set the field.

 You may want to test for a NIL instead of using Assign in any case because it looks like this isn't a class or object method you are testing against..

 Also, callbacks when done from some outside of the control of the program should be done in a Class static address.