[fpc-pascal] casting interfaces and objects

[Marc Santhoff]

Am Sonntag, den 18.05.2008, 23:07 +0200 schrieb Marco van de Voort:
> > I have two questions about interface usage.
> > 
> > 1. How interfaces are to handle when casting?
> > Theoretically it should be safe to do this:
> > 
> > TObservable = class
> > public
> >   procedure Register( obsv: IObserver );
> > private
> >   procedure NotifyObservers(param: TParameter);
> >   fObservers: TFPObjectList;
> > end;
> > 
> > procedure TObservable.Register( obsv: IObserver );
> > begin
> > 	fObservers.add(TObject(obsv));
> > end;
> 
> No it is not safe. The interface points to an interface table, and you cast
> it if it were a class VMT.

I see. I indeed had a somewhat "hacky" feeling using that cast ...

Since the compiler dod not warn about using the "as" operator I assume
it is safe? Like this:

procedure TObservable.DeRegister( obsv: IObserver );
var
	i: integer;
begin
	if (fObservers.count<=0) then exit;
	for i:=0 to fObservers.count-1 do begin
		if ((fObservers[i] as IObserver)=obsv) then begin
			fObservers.delete(i);
			fObservers.pack;
			exit;
		end;
	end;
end;

(Although this example will no longer exist when using an
T/IInterfaceList.)

> Have a look at IInterfaceComponentReference and use/implement the
> getcomponent method. Note that this was added post-2.2.0

I'll have a look and put it on my todo-list.

Thanks a lot,
Marc