Re: A bad counterintuitive behaviour of Ada about OO

[Maciej Sobczak <see.my.homepage@gmail.com>]

On Friday, August 8, 2014 3:22:56 PM UTC+2, Dmitry A. Kazakov wrote:

> >> I still do not understand the example.
> 
> > http://en.wikipedia.org/wiki/Covariant_return_type
> 
> type A is tagged null record;
> function getFoo return A;

Wrong. getFoo should have a factory as a parameter (in the Wikipedia example this is hidden as "this" parameter). More complete example would be:

   package Products is
      type A is tagged null record;
      type B is new A with null record;
   end Products;
   
   package Factories is
      type A_Factory is tagged null record;
      function Make (AF : in A_Factory) return Products.A;

      type B_Factory is tagged null record;
      overriding function Make (BF : in B_Factory) return Products.B;
   end Factories;

and GNAT says:

   subprogram "Make" is not overriding
   not type conformant with declaration at line XXX
   return type does not match

As Randy pointed out, this cannot work in Ada. It's not even a matter of "fixing" something.

> No dispatch, no covariance.

Feel free to make up your own definitions.

> If you are in the hierarchy of the factory
> type, there cannot be any covariance in an unrelated hierarchy.

This is *exactly* the definition of covariance, at least the one I know:

http://en.wikipedia.org/wiki/Covariance_and_contravariance_%28computer_science%29#Covariant_method_return_type

> The only case where this may happen is parallel type hierarchies when two
> types are inherited synchronously, e.g.
> 
>    type Object is tagged ...
>    type Handle is tagged ...
> 
>    function Access (X : Handle_To_Object) return Object;
> 
>    type New_Object is new Object with ...
> 
> must produce
> 
>    type New_Handle is new Handle with ...
> 
> and
> 
>    function Access (X : New_Handle) return New_Object;

Yes.

> No OOPL I know can handle this.

Have you heard about those niche and unpopular languages like C++ and Java? ;-)

-- 
Maciej Sobczak * http://www.msobczak.com * http://www.inspirel.com