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From: "Dmitry A. Kazakov" <mailbox@dmitry-kazakov.de>
Newsgroups: comp.lang.ada
Subject: Re: Dispatching to a common most special ancestor
Date: Thu, 12 Jun 2003 08:08:02 +0200
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Date: 2003-06-12T08:08:02+02:00
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Stephan Heinemann wrote:

> Please have a look at the following code:

[...] 

> I wanted Equals to be dispatched to the common object ancestor but
> instead a constraint error is raised. How might I resolve this?

There is a work-around for multiple dispatch. You might look at
http://www.dmitry-kazakov.de/ada/components.htm which presumably does what 
you want. In short, you declare:

function Equal
         (  Left  : Object;
            Right : Object'Class;
            Flag  : Boolean := False
         )  return Boolean;
function Less
         (  Left  : Object;
            Right : Object'Class;
            Flag  : Boolean := False
         )  return Boolean;

and then override, for instance Less:

function Less
         (  Left  : A_New_Object;
            Right : Object'Class;
            Flag  : Boolean := False
         )  return Boolean is
begin
   if (  Flag
      or else
         Right not in A_New_Object'Class
      or else
         Right in A_New_Object
      )
   then
      -- Implement it here
      ...
   else
      -- Dispatch on the second parameter
      return
         not (  Less (Right, Left, True)
             or else
                Equal (Right, Left, True)
             );
   end if;
end Less;

The idea is that you do something if Left :> Right, otherwise you redispatch 
through Right. For a commutative operation it is pretty straightforward. 
For a non-commutative (like Less) it is a little bit tricky, as the code 
shows.

Beware, that the predefined "=" cannot be disallowed, so you cannot do

function "=" (Left : Object; Right : Object'Class) is
begin
   return Equal (Left, Right);
end "=";

because it would just overload the predefined one.

-- 
Regards,
Dmitry A. Kazakov
www.dmitry-kazakov.de