[fpc-devel] Propose: change TComponent.Tag from longint to PtrInt

[Burkhard Carstens]

Am Mittwoch, 6. September 2006 11:05 schrieb Michael Van Canneyt:
> On Wed, 6 Sep 2006, Ale Katona wrote:
> > The problem here is that if you introduce a changing type to Tag,
> > you're going to break streaming of the components between various
> > platforms. However I think simplest would be to simply make the tag
> > something huge for years to come, like Int64.
> >
> > The idea with hidden union is nice tho. But you'll need to know how
> > to stream it precisely and there's also endianness to take care of
> > then.
>
> I've been following the discussion, and the tag property will remain
> what it is: a 32-bit integer. It was introduced by Delphi to mimic
> the Tag property which exists in VB, to allow it to be used in case
> statements. Changing it will probably cause more problems than it
> will solve.
>
> In casu:
>
> Typecasting 'Tag' to a pointer is a non-portable, ugly hack and we
> should not encourage such hacks.
>
> If you want to store objects, use a TCollection, and store
> TCollectionItem.ID in the component tag. ID is a unique identifier,
> even when elements are added and removed from the collection.
>
> Slightly more work, but clean and portable.
>
> Michael.

In many situations, the collection would be overkill, so I'd also opt 
for the new public property "UserData : pointer" or better "TagPtr : 
pointer". This would make it harder to create code, that also compiles 
with delphi, but that doesn't bother me ;-)

Anyway, if one needs to compile with delphi, one could continue to use 
pointer(tag), as delphi is 32-bit only anyway ...
This way the user has the choice to write code that compiles with 
fpc32+delphi or fpc32+fpc64

regards
 Burkhard