[fpc-devel] problem with "is" operator

Tom Verhoeff T.Verhoeff at tue.nl
Mon Apr 18 10:29:08 CEST 2005

On Sun, Apr 17, 2005 at 12:01:36PM -0700, Linuxer Wang wrote:
> Can anybody tell me how can I know which specific type an instance of
> class is? The "is" operator seems weird when interface is used.
> Suppose TMyInterface is a interface, and classes TCircle and TSquar
> both implements TMyInterface, and inst:TMyInterface, inst :=

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.

> 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.

E-MAIL: T.Verhoeff @ TUE.NL     | Fac. of Math. & Computing Science
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