[fpc-pascal] Interface type error

[Sven Barth]

Ryan Joseph via fpc-pascal <fpc-pascal at lists.freepascal.org> schrieb am
So., 2. Feb. 2020, 03:37:

> Why doesn't this compile? IClassName2 descends from IClassName1 so
> shouldn't TClassName be compatible with IClassName1?
>
> ========================================
>
> {$mode objfpc}
> {$interfaces corba}
>
> program test;
>
> type
>   IClassName1 = interface
>   end;
>   IClassName2 = interface(IClassName1)
>   end;
>
> type
>   TClassName = class(IClassName2)
>   end;
>
> procedure Pass(int: IClassName1);
> begin
>
> end;
>
> var
>   c: TClassName;
> begin
>   // Incompatible type for arg no. 1: Got "TClassName", expected
> "IClassName1"
>   Pass(c);
> end.
>

As I had explained in the other thread some months ago interfaces in Object
Pascal mean literally that the type can be cast to the specified
interfaces. Parent interfaces are *not* part of this. If you want this you
need to first cast the class to IClassName2, then the "interface
inheritance" mechanism can take over.

Regards,
Sven

>
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