[fpc-devel] Parameters must match exactly?

[Mattias Gaertner]

On Mon, 17 May 2010 10:05:27 +0100
Martin <fpc at mfriebe.de> wrote:

> On 14/05/2010 15:52, Mattias Gaertner wrote:
> > "var" means, you can do this:
> >
> > type
> >    TQStrings = class(TStrings)
> >    end;
> >
> > procedure A(var s: TStrings);
> > begin
> >    s:=TQStrings.Create;
> > end;
> >
> > Then your TStringList will no longer be a TStringList. That's why 'var'
> > must match exactly.
> > Same for out.
> > Const and no modifier allows to pass descendants.
> >    
> 
> yes, true, then if we follow that consequently:
> 
> 
> procedure Foo(out s: TStrings);
> begin
> end;
> 
> var x: TObject;
> foo(x);
> 
> should work? (or well only if it is made sure, that out really means 
> out, so the procedure is not allowed to use the value that was passed in)
>
> "s" can be assigned a TStrings (or descendant of) => which should be 
> fine, since it can be stored in x too.

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.


Mattias