[fpc-pascal] Interface delegates and the implements property specifier

Sven Barth pascaldragon at googlemail.com
Sat Dec 21 22:32:02 CET 2019

Am 21.12.2019 um 16:49 schrieb Adriaan van Os:
> Section 7.4 Interface delegation 
> <https://www.freepascal.org/docs-html/current/ref/refse45.html#x98-1200007.4> 
> of the FPC Language Reference Guide discusses interface delegates and 
> the implements property specifier.
> For example
> $interfaces corba}
> type
>   IMyInterface = interface
>     procedure P1;
>   end;
>   IMyInterface2 = interface
>     procedure P2;
>   end;
>   TMyClass = class(TInterfacedObject,
>                    IMyInterface, IMyInterface2)
>     FI2 : IMyInterface2;
>   protected
>     procedure IMyInterface.P1 = MyP1;
>     procedure MyP1;
>   public
>     property MyInterface: IMyInterface2
>        read FI2 implements IMyInterface2;
> I had hoped that procedure IMyInterface2.P2 would now be visible as a 
> method of TMyClass. This would be quite helpful in implementing 
> multiple-inheritance. But no, the implements specifier completely 
> hides it. I assume this has been discussed before.

Yes, this is by design, because the idea of the interface delegation is 
that the class can be cast to the designated interface type (thus 
supporting the interface) and then be used as if it directly implemented 
the interface.

> However (after adding an identifying string to IMyInterface and 
> IMyInterface2), TObject.GetInterface( IMyInterface2) fails for the FI2 
> delegate but succeeds for an object of TMyClass. This seems 
> contradictory, as IMyInterface2 is really part of the F12 delegate, 
> not of TMyClass.

I don't get what you're saying here. Would you please provide code that 
illustrates your problem?


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