[fpc-devel] Is this supposed to work (generic)?

Kostas Michalopoulos badsectoracula at gmail.com
Tue Mar 28 20:29:29 CEST 2023

On 3/26/23 14:30, Martin Frb via fpc-devel wrote:
 >    generic GenLinkedList<D, T> = class
 >      Data: D;
 >      Next: T;
 >    end;
 >    TSome = class;
 >    TSome = class(specialize GenLinkedList<integer, TSome>);

AFAIK "specialize" essentially "pastes" the parameters (after some 
checking) so it'd be as if you had declared...

     TSome = class;
     GenLinkedList = class
       Data: Integer;
       Next: TSome;
     TSome = class(GenLinkedList);

...which is perfectly valid Free Pascal code.

Also AFAIK the reason the others do not work is because when specialize 
is called, TFoo and TBar are not known to the compiler yet, but the 
forward declaration with TSome tells to the compiler that it is a class.

I guess it *is* a bit inconsistent that you can use the name of the type 
you are declaring during the declaration itself for classes - like "TFoo 
= class Foo: TFoo; end" - or target a type with a pointer before the 
targeted type is even known and yet specialization needs to know ahead 
of time everything, but that's how things are :-P


More information about the fpc-devel mailing list