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From: Matthew Heaney <matthew_heaney@acm.org>
Subject: Re: Pb with use of redefined "=" operator
Date: 1998/11/04
Message-ID: <m3zpa860yd.fsf@mheaney.ni.net>#1/1
X-Deja-AN: 408162920
Sender: matt@mheaney.ni.net
References: <363F62F3.3FF7@club-internet.fr>
NNTP-Posting-Date: Tue, 03 Nov 1998 19:17:13 PDT
Newsgroups: comp.lang.ada
Date: 1998-11-04T00:00:00+00:00
List-Id: <comp.lang.ada>

Fran�oise & Herv� BITTEUR <hbitteur@club-internet.fr> writes:

> I just run in a problem when combining :
> - Ada.Strings.Bounded where '=' operator is redefined,
> - A so far correct generic package (old Ada83 code) importing a private
> (non-limited) type.
> 
> The problem is that any instantiation of the generic package will use
> the "predefined" equality operator provided with any non-limited type,
> rather than the specific definition related to Ada.Strings.Bounded.
> Which leads to unexpected results on some occasions.

This phenonmenon is called "reemergence of predefined operators," and
it's one of Ada's trap doors.  When you import a (non-tagged) type as a
formal type, without also explicitly importing operators, then the
predefined operators for the type are used, irrespective of whether the
actual type has redefined them.


> I have attached a very simplified example to illustrate the situation.
> An obvious fix is to modify the spec of the generic package to
> explicitly import a definition of '=' :
>    with function "=" (L, R : in Item) return Boolean is <>;
> And the redefined version if any (or the predefined one if none) will
> then be used.


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 ...


> But what puzzles me is that :
> 1/ No warning is raised by the compiler (how could it be otherwise ?)

Unfortunately, this is a "feature" of the language, not a flaw.

> Would you thus recommend that the line above ("with function "=" ...) be
> systematically added to each and every existing generic package using
> non-limited parameters ?

Yes.  Always.