Re: Merits of re-dispatching [LONG]

[dmitry@elros.cbb-automation.de (Dmitry A. Kazakov)]

On Thu, 7 Feb 2002 20:06:42 -0500, "Matthew Heaney" <mheaney@on2.com>
wrote:

>"Dmitry A. Kazakov" <dmitry@elros.cbb-automation.de> wrote in message
>news:3c62524f.93369796@News.CIS.DFN.DE...
>>
>> package Self_Identifying is
>>    type A is new
>>       Ada.Finalization.Limited_Controlled with private;
>>    type A_Ptr is access all A'Class;
>>    procedure Finalize (Object : in out A);
>>    procedure Foo (Object : in out A);
>> private
>>    type A is new
>>       Ada.Finalization.Limited_Controlled with
>>    record
>>       Self : A_Ptr := A'Unchecked_Access; -- Class wide self-reference
>>    end record;
>> end Self_Identifying;
>
>This is a horrible way to do this.  Don't use a named access type when an
>access discriminant will do:
>
>  type Handle_Type (O : access A'Class) is limited null record;
>
>  type A is new Limited_Controlled with record
>    H : Handle_Type (A'Access);
>  end record;

I like it, but it has its own drawbacks. If only the component H
should be exposed and all others be hidden, then one should make a lot
of equilibristic:

--
-- Public part of A
--
   type A_Public;
   type A_Handle (O : access A_Public'Class) is
      limited null record;
   type A_Public is abstract new
      Ada.Finalization.Limited_Controlled with
   record
      H : A_Handle (A_Public'Access);
   end record;
   procedure Foo (Object : in out A_Public) is abstract;
--
-- Full A
--
   type A is new A_Public with private;
   type A_Ptr is access all A'Class;
   procedure Foo (Object : in out A);

private
   type A is new A_Public with record
      ... -- private components;
   end record;

But this is another question of course.

>> Is there examples where re-dispatching is really unavoidable?
>
>There is a very simple solution to your problem: don't do it.

So there is no such examples! (:-))

Regards,
Dmitry Kazakov