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From: Mats Weber <Mats.Weber@elca-matrix.ch>
Subject: Re: Q: Primitive operation of a type
Date: 1997/07/02
Message-ID: <33BA43CC.E27C69CC@elca-matrix.ch>#1/1
X-Deja-AN: 254047108
References: <5oq51h$t7u@netline.jpl.nasa.gov>
 <mheaney-ya023680000107972317060001@news.ni.net>
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Organization: ELCA Matrix SA
Reply-To: Mats.Weber@elca-matrix.ch
Newsgroups: comp.lang.ada
Date: 1997-07-02T00:00:00+00:00
List-Id: <comp.lang.ada>


Matthew Heaney wrote:

> This will dispatch on the tag of the O1 and O2, but only if O1 and O2
> have
> the same tag, which can't be determined until run-time.  If the
> tag-check
> fails, then an exception is raised (I forget which, either
> Constraint_Error
> or Program_Error).

It's Constraint_Error.

> procedure P (O1, O2 : T1'Class) is
> begin
>    if O1 = O2 then
> 
> This will dispatch on the equality operator, if the tags are the same.
> If
> the tags are different, then obviously the objects can't be the same,
> so no
> exception is raised (thankfully) and the function just returns False.

I'm not so sure we should be thankful for this one. In the following
situation, for instance, you may want "=" to return True even it the tags are different:

   type Root_Set is abstract tagged private;

   type AVL_Tree_Set is new Root_Set with private;

   type Boolean_Array_Set is new Root_Set with private;

   A : AVL_Tree_Set;
   B : Boolean_Array_Set;

   procedure P (X, Y : Root_Set'Class) is 
   begin
      if X = Y then ...
   end P;

   P(A, B);

now X => A and Y => B have different tags, but I would like "=" to return True
iff A and B have the same elements, not the same elements and implementation.