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From: Mats Weber <Mats.Weber@elca-matrix.ch>
Subject: Re: Pb with use of redefined "=" operator
Date: 1998/11/04
Message-ID: <3640B520.D7BEEE72@elca-matrix.ch>#1/1
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References: <363F62F3.3FF7@club-internet.fr> <m3zpa860yd.fsf@mheaney.ni.net>
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Organization: ELCA Matrix SA
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Reply-To: Mats.Weber@elca-matrix.ch
Newsgroups: comp.lang.ada
Date: 1998-11-04T00:00:00+00:00
List-Id: <comp.lang.ada>

Matthew Heaney wrote:

> ALWAYS import equality for any non-tagged formal type.  In the data
> structure library I'm building, every private type is imported as
> 
> generic
> 
>    type Item_Type is private;
> 
>    with function "="
>      (L, R : Item_Type)
>      return Boolean is <>;
> ...
> package ACL.Stacks.Adapters is ...

I would advise against this, because predefined equality will reemerge
in other situations, such as this:

    type Pair is 
       record
          X, Y : Item_Type;
       end record;

"=" (Pair, Pair) will use the predefined equality on Item_Type, so that
you have to write a new "=" function for Pair. This makes it very hard
to check that the code is correct (i.e. every record/array declaration
that contains a generic formal type must have its own redefined "=").

My advice is that you must know about the reemergence problem and never
instantiate a generic with a type whose predefined "=" does not work.
You can often do this it using tagged types.