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From: Ludovic Brenta <ludovic@ludovic-brenta.org>
Newsgroups: comp.lang.ada
Subject: Re: Ada vs Eiffel - Ada programmer approach
Date: Thu, 4 Jun 2009 02:02:01 -0700 (PDT)
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Date: 2009-06-04T02:02:01-07:00
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Yannick Duch=EAne wrote on comp.lang.ada:
> On 26 mai, 20:21, "Jeffrey R. Carter"
>
> <spam.jrcarter....@nospam.acm.org> wrote:
> > I agree. I never create tagged types, except to obtain finalization. I =
think the
> > widespread acceptance of programming by extension, even by People Who S=
hould
> > Know Better, represents the triumph of those who emphasize ease of writ=
ing over
> > ease of reading.
>
> So how do you do extentions ?
> Did you never need this ?

The question is not really "how do you do extension", but "why do you
do extension". In my experience, composition is usually a more correct
choice. There are, of course, some cases where extension is good but
my inheritance hierarchies are usually very shallow.

For example, if you want some record type T to become controlled, you
don't have to make extend Ada.Finalization.Controlled; you don't even
have to make T tagged. You can use a mixin instead:

with Ada.Finalization;
package P is
   type T is private; -- note: not publicly tagged
private
   type Controller_T (Enclosing : access T) is
     new Ada.Finalization.Controlled with null record;
   overriding procedure Initialize (C : in out Controller);
   overriding procedure Adjust (C : in out Controller);
   overriding procedure Finalize (C : in out Controller);
   type T is record -- note: not privately tagged, either.
      Controller : Controller_T (Enclosing =3D> T'Access);
      -- other components...
   end record;
end P;

The primitive subprograms Initialize, Adjust and Finalize can see the
entire enclosing object through the discriminant of C.

--
Ludovic Brenta.