[fpc-devel] Parameters must match exactly?

[Mattias Gaertner]

On Mon, 17 May 2010 10:40:12 +0100
Martin <fpc at mfriebe.de> wrote:

> On 17/05/2010 10:29, Mattias Gaertner wrote:
>[...]    
> > Theoretically: yes, this could work for classes.
> > But as Jonas explained a few times: 'out' is *not* write only.
> > Out is finalised and must therefore match exactly.
> > And even write-only is not enough for "object"s because an ancestor
> > has too less memory.
> >    
> 
> Hm, finalization isn't happening in the user code. So shouldn't be an 
> issue... and:
> - are class-instance finalized? (maybe with interfaces)
> - it's an implementation detail of the compiler => it could (maybe does) 
> happen, in the calling code (where type info is known)
> 
> memory is allocated on the heap
> s and x are both just pointers => so the size matches
> Actually x is a pointer to s, and s a pointer to the memory. but s can 
> be changed, so the memory can be re-allocted.

No, for example:

type
  TAncestor = object
    i: string;
  end;

  TMyObject = object(TAncestor)
    j: string;  // need more mem
  end;

procedure Doh(out o: TMyObject);
begin
  o.j:='3'; // writing beyond
end;

procedure A;
var
  o: TAncestor; // limited mem
begin
  Doh(o);
end;


Mattias