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From: Adam Beneschan <adam@irvine.com>
Newsgroups: comp.lang.ada
Subject: Re: Exceptions and out procedure arguments (using GNAT GPL)
Date: Tue, 19 Jun 2007 08:21:26 -0700
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Date: 2007-06-19T08:21:26-07:00
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On Jun 19, 12:34 am, Maciej Sobczak <see.my.homep...@gmail.com> wrote:
> On 19 Cze, 07:23, Fionn Mac Cumhaill <invisi...@hiding.from.spam>
> wrote:
>
> > I eliminated the problem by modifying the offending procedure to not
> > raise exceptions. It now returns a status code in an additional out
> > argument.
>
> You might also introduce a temporary variable in the procedure to keep
> the "candidate" result and copy it into the relevant out parameter
> just before leaving the procedure (on the successful path).
> This would be better than changing the interface and error handling
> policy, which is very intrusive.

But it might be the correct design anyway.  My gut feeling is that, in
the abstract, a subprogram should either produce a result *or*
(perhaps) raise an exception, but not both; in general, if your
definition of a subprogram is that, under certain conditions, the
subprogram will raise an exception AND the caller can expect a certain
value to be returned (whether in an OUT parameter or an IN OUT or a
global or in something pointed to by an access parameter or whatever)
even though an exception is raised, the design is wrong.  It's better
to use an OUT status code of some sort in that case.  Sorry if I'm not
explaining this well---I don't always know what the common terminology
is for the things I'm thinking.  Also, I'm not saying that this
necessarily applies to Fionn's program.

                     -- Adam