OOP_(re)

Wed, 28 May 1997 15:46:06 -0400 (EDT)

On Wed, 28 May 1997, The African Chief wrote:

> >Well, this is *exactly* what object-orientation is there for, so you add
> >the fields when you need them. TObject itself is a "do-nothing" object. As
> >such, it is *not* windowed and should *not* have any Handle field as it
> >won't ever have ANYTHING in it! A TWindow descendent shall add this field.
> 
> Yes (but Handle is present in Delphi as well). However, the main reason
> for keeping it is because the OBJECTS unit exports a function which returns
> a pObject, which can be used to locate an object instance by its Handle
> (i.e., given any Handle, you would thereby be able to locate its owner). 
> This can also be done via the object's SelfID (see below) in cases where
> the SelfID and Handle are different (which would invariably be the case).

Maybe the InstanceFromSelfID is valid (though I'd simply use the instance
pointer as a handle), but (in a Turbo Vision context) only TView should
have a Handle, because it is the first class that can possibly have a
window handle. The InstanceFromHandle would hence return a PView or a
PWindow, which is much more useful, since TView/TWindow has methods for
window manipulation that TObject doesn't even have.

> This is not an academic issue, because I have seen the utility of 
> this facility in other contexts (viz - my portable "CHIEFAPP" class
> library, for BP 7.x, Delphi1 and Delphi2, from which project this
> OBJECTS unit was borrowed and cannibalised). Thus, the fields
> need to be in the ultimate "mother of all objects".

I think you could have done the very same (maybe even cleaner regarding
the design) without adding any special fields to TObject.

> It has a present use - to be able to locate every object instance by its 
> own unique ID. See above.

The instance pointer is an unique ID, why have another redundant one?

Pierre Phaneuf

"The use of COBOL cripples the mind; its teaching should, therefore, be
regarded as a criminal offense." - Edsger W. Dijkstra.

Pierre Phaneuf (pp@dilu.ml.org)