[Solved]Generic methods in interfaces - No go?!?

[cdbc]

Hi
I'm testing generic methods, they work fine.
So I wanna put them in an 'interface', but that sent the compiler running and screaming... "Interfaces can't have fields!" - Hmmm, what's that all about, it's a method?!?

Code: Pascal  [Select][+][-]

1.  {
2.   IgenFactory = interface(IInterface)['{237FE756-12DE-4D6E-86C3-CA487F32BCB5}']
3.    generic function PickSvc<T>(const aName: string): T;
4.  end;                                                         }
5.  { TgenFactory }
6.  TgenFactory = class(TInterfacedObject{,IgenFactory})
7.  public
8.    generic function PickSvc<T>(const aName: string): T;
9.  end;

The implementing class works fine, no worries, but if I uncomment the interface, the compiler instantly complains "Interfaces can't have fields!".
?!? Me No Understand ?!? 
Regards Benny

[Martin_fr]

I guess its the wrong error message...

Starting at

Code: Pascal  [Select][+][-]

1.   TFoo = class
2.     generic procedure Bar<T>; virtual;
3.   end;
4.  

gives: project1.lpr(8,38) Error: Generic methods can not be virtual

And it makes sense.

1) Virtual methods must be in the VMT.
2) The size and order of the VMT must be known (per class) and not change.

The 2nd means, imagine
- The VMT has size=10 in the unit that defines that class
- another unit_X adds a specialization at position 11
- another unit_Y also adds a specialization, it does not know about unit_X, so also  at position 11.
- Or an inherited class already has put its virtual method at position 11
...
Then if code tries to use this method, only one can be at pos 11.


Methods of interfaces are handled like virtual methods. they are listed in a table and accessed by position.
So if my understanding is right, they can not (as in: are technically not possible to) be generic.

Just the error message seems an odd choice....

[Thaddy]

Yes, you are right.
Interfaces can not be kamelions. They must be fully defined.

[cdbc]

Hi
Thank you, the both of you 
Now that's cleared up, I can move on to "the next live picture", so to speak 
Makes perfect sense.
Regards Benny