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Newsgroups: comp.lang.ada
Subject: Re: Is the Writing on the Wall for Ada?
References: <mailman.20.1062997816.295.comp.lang.ada@ada.eu.org>
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From: Matthew Heaney <matthewjheaney@earthlink.net>
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olehjalmar kristensen - Sun Microsystems - Trondheim Norway <ok145024@sun.com> writes:

> Beats me. In fact, one of the *nice* things about the C++ multiple
> inheritance is that it allows me to express roles (a la Reenskaug) and
> then combine different roles in the same object without jumping
> through hoops. In Smalltalk you need an extra tool to do this, in C++
> all you need is a little care. I suppose it *could* be done in Ada
> with generics, but I have not tried it, and it would certainly be more
> obfuscated.

The mechanism for implementing roles (what we call "views") in Ada95 is
access discriminants, not MI.

For example, if I want to have a "persistence role," I can do this:

package Persistence_Views is

   type Root_Persistence_Type is
      abstract tagged limited null record;

   type Persistence_Class_Access is
     access all Root_Persistence_Type'Class;

   procedure Write
     (Stream : in out Root_Stream_Type'Class;
      Persistence : access Root_Persistence_Type) is abstract;

   procedure Read
     (Stream : in out Root_Stream_Type'Class;
      Persistence : access Root_Persistence_Type) is abstract;

   procedure Save
     (Persistence : access Root_Persistence_Type'Class);
   
end Persistence_Views;


To add support for persistence to a type, you can do this:

with Persistence_Views;
package P is

   type T is tagged limited private;

   function Persistence (O : access T) 
     return Persistence_Class_Access;

   ...
private

   type Persisence_Type (O : access T) is
     new Root_Persistence_Type with null record;

   procedure Write (...);
   procedure Read (...);

   type T is tagged limited record
     Persistence : aliased Persistence_Type (T'Access);
     ...
   end record;

end P;

 
So now given an object of type T (which supports persistence), you can
oo this:

procedure Op (O : in out T) is
begin
   Save (Persistence (O'Access));
end;


We've shown one role here, but in fact any number of views may be
supported by a type:

procedure Op (O : in out T) is
begin
   View_1_Op (View_1 (O'Access));
   View_2_Op (View_2 (O'Access));
   View_3_Op (View_3 (O'Access));
   ...
end;


Contrary to being "obfuscated," implementing roles in Ada95 is quite
simple and elegant.  And no MI is required.