[fpc-devel] interfaces / inheritance / again -

[Michael Van Canneyt]

On Mon, 2 Feb 2026, Martin Frb via fpc-devel wrote:

>  Not yet tested, but only matters if it is not going to be discontinued 
> (in case it actually works)
>
> I want a
>   generic Foo <_A: SomeIntf;  _B: TBar>
>
> such as that I can give an inherhited interface.
>
> The interface has
>   GetBar => which return TBar => or subclass....
> Hence for the generic I have a param to specify the class.
>
>
> But, I cans do
> IFoo = interface
>   proceduer GetBar: TBar;
>
> ISubFoo = interface(Foo)
>   proceduer GetBar: TSubBar;

Are you deliberately trying to shoot yourself in the foot here ?
(better your foot than my foot obviously...)

>
> However, (tests pending) if the below works, would it be working by 
> indent? And therefore not removed by some bug fix in future?
>
> IFoo = interface
>   proceduer GetBar: TBar;
>   property Bar: TBar;

I assume this should be:

property Bar: TBar read GetBar;

>
> ISubFoo = interface(Foo)
>   proceduer GetSubBar: TSubBar;
>   property Bar: TSubBar;

Similarly, I assume you mean

  property Bar: TSubBar read GetSubBar;

property overrides exist, but they must keep the same type, they can only
change the specifiers. So I don't think this will compile. If it does, it
is probably by accident.

> The property is just syntactic sugar isn't it?
> But the generic would see it according to the actual type of the gen param.

I don't know, but to me it looks like a definite way to confuse yourself and others.

I would define this as

   ISubFoo = interface(Foo)
     procedure GetSubBar: TSubBar;
     property SubBar: TSubBar;
   end;

Michael.