> 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 _______________________________________________ fpc-devel maillist - fpc-devel@lists.freepascal.org http://lists.freepascal.org/mailman/listinfo/fpc-devel
