Re: troubles learning OOP: expected type "Parent.Some_Child", found type Parent'Class

["Dmitry A. Kazakov" <mailbox@dmitry-kazakov.de>]

On 2018-02-12 19:17, Mehdi Saada wrote:
> ... Somehow, it doesn't surprise me anymore.
> 
>> Get_Elem take T_Token'Class
> How could I do that, since it meant to RETURN a different type for each subclass !

1. Class /= type. Class is a set of types rooted in a type.

2. T'Class is meant to have values of/equivalent to values of all these 
different types.

3. You can always convert a member of the class (a specific type) to the 
  class-wide type of any class this specific type belongs to.

   type T is ...;
   type S is new T with ...;

   function Foo return T'Class is
      Result : S;
   begin
      return T'Class (Result); -- This is OK
   end Foo;

> Now, I know why I prefered mutant types even before using them.

Mutable instances are always less safe than immutable ones.

-- 
Regards,
Dmitry A. Kazakov
http://www.dmitry-kazakov.de