[fpc-devel] (ref types / circles) Re: Defer keyword

[Ryan Joseph]

> On May 9, 2021, at 3:40 AM, Sven Barth <pascaldragon at googlemail.com> wrote:
> 
> === code begin ===
> 
> {$mode objfpc}
> 
> type
>   TTest = class
>   protected
>     procedure DoSomething;
>   end;
> 
>   TTestSub = class refcounted(TTest)
>   public
>     procedure Test;
>   end;
> 
> procedure TTest.DoSomething;
> begin
>   // maybe this functions stores the reference
>   SomeFuncThatTakesAObject(Self);
> end;
> 
> procedure TTest.Test;
> begin
>   DoSomething;
> end;
> 
> === code end ===

I see, the reference counting is broken because you move up into a non-ref counted class. Yeah that's something programers simply should not do or be prevented from doing. I don't see this particular case being a problem however because your ref counted object is going to be in the base of a hierarchy, probably enforced even. The only reason for opt-in ARC is so we don't pollute TObject but it still doesn't mean that  you should be adding this in the middle of class trees.

Here is the bigger problem:

var
  list: TObjectList;

procedure HandleObject(obj: TObject);
begin
   // the list now stores the class but it's lost ref-counting because it was cast to TObject
   list.Add(obj);
end;

var
  obj: TTestSub;
begin
  HandleObject(obj);
end;

or

var
  obj: TObject;
begin
  // we  lost ref counting now!
  obj := TTestSub.Create;
  HandleObject(obj);
end;

Once you cast away from your managed class type things fall apart. Records aid this by not allowing casting but you could enforce some kinds of checks for managed classes if you wanted to. Doesn't seem like a deal breaker to me if you add new type rules for passing/assigning.

Regards,
	Ryan Joseph