Re: Exceptions and out procedure arguments (using GNAT GPL)

["Dmitry A. Kazakov" <mailbox@dmitry-kazakov.de>]

On Tue, 19 Jun 2007 08:21:26 -0700, Adam Beneschan wrote:

> 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.

I don't think this is a good advice. In my view a right design assumes that
whether an exception is propagated or not, the subprogram should not leave
anything in an undefined state.

That is independent on the way an in-out parameter is passed. If the
parameter is by-reference, then the subprogram shall document all side
effects on it, which cannot be rolled back.

(Thee is a quite specific case of non-initialized out parameters, but I
don't like the idea of using that pattern rather than result anyway.)

-- 
Regards,
Dmitry A. Kazakov
http://www.dmitry-kazakov.de