[fpc-devel] problem with "is" operator

Uberto Barbini uberto at ubiland.net
Mon Apr 18 20:09:51 CEST 2005

> It does not seem right to declare var inst: TMyInterface if you
> want inst to have circles and squares as values.  I would expect
> that you also have a class TFigure, of which TCircle and TSquare
> both are descendants.  These could also implement TMyInterface.
> You then declare var inst: TFigure and can do inst := TCircle.Create.

If so, why bother with Interfaces?
You need interfaces exactly when you cannot define a common ancestor for two 
classes with similar behavior.

The classical example is IFly implented by TBat and TSwallow.

> > TCircle.Create. How can I know which instance is inst? The following
> > code can not even compile:
> >
> > if inst is TCircle then ...
> >
> > class type expected, but got "TMyInterface"
> > Incompatible type for arg no. 2: Got "TMyInterface", expected "TObject"
> The type of inst needs to be a class type (such as TFigure above), not
> an interface object type.  If you do as I suggest above, it should work.

if inst is TCircle then ...

I don't think this can possibly work in fpc or Delphi. 
Apart tecnical problems, it'd be plainly wrong: using interfaces you lost the 
rights to know the original class.

Sometimes I added a GetUnderObject() to my interfaces to get the actual 
object. But it's a choice up to the interface author.
BTW I needed it to release the object through the interface.
I suspect that if you shouldn't ever need to know the actual class when using 
interfaces (maybe apart debug reasons).

Bye Uberto

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