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Subject: Re: Is the Writing on the Wall for Ada?
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From: Matthew Heaney <matthewjheaney@earthlink.net>
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Dmitry A. Kazakov <mailbox@dmitry-kazakov.de> writes:

> (**) BTW a great unsolvable problem in both Ada and C++, both with MI
> or without, is to make a container of a subtype a subtype of the
> container of the base type. Which highlights present problems with ADT
> and *helpless* attempts to solve this using templates.

Au contraire.  The *simplest* way to solve that problem is using
templates.

package P is
   type T is tagged null record;
   procedure Op (O : in out T);
end P;

package P.C is
   type NT is new T with null record;
   procedure Op (O : in out T);
end;


function "<" (L, R : P.T) return Boolean is ...;

package T_Sets is
   new Ada.Containers.Sets.Sorted.Unbounded (P.T);

function "<" (L, R : P.C.NT) return Boolean is ...;

package NT_Sets is
   new Ada.Containers.Sets.Sorted.Unbounded (P.C.NT);

  T_Set : T_Sets.Container_Type;
  NT_Set : NT_Sets.Container_Type;

Now let's say you want an algorithm that can operate over the items in
either of the set containers declared above.  After all, that why you
wanted NT_Set to be a subtype of T_Set.  So let's just abstract-away the
container:

   generic
      type Iterator_Type is private;
      with procedure Succ (Iter : Iterator_Type)
         return Iterator_Type is <>;
      function To_T (Iter : Iterator_Type)
         return T_Class_Access;
   procedure Generic_Algorithm
     (First, Back : Iterator_Type);

Now I can do this:

   function To_T is
      new T_Sets.Generic_Element (T_Class_Access);  --check this

   function To_T is
      new NT_Sets.Generic_Element (T_Class_Access); --ditto

And now I do this:

   procedure Algorithm is
     new Generic_Algorithm (T_Sets.Iterator_Type);

   procedure Algorithm is
     new Generic_Algorithm (NT_Sets.Iterator_Type);

And so now I can do this:

   Algorithm (First (T_Set), Back (T_Set));
   Algorithm (First (NT_Set), Back (NT_Set));

Problem solved.  No MI or base types or inheritance or interfaces or any
other crap is needed.  Generics work just fine thank you very much.