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From: "Matthew Heaney" <matthew_heaney@acm.org>
Subject: Re: 'constant functions' and access constant params (was Re: Array of
 Variant Records Question...)
Date: 1999/09/23
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In article <37E9B259.7C58@l5i.net> , Vincent Marciante <ma4rciant@l5i.net>
wrote:

> Would the following (or similar) also require consideration?
> Doesn't it also exhibit a loophole?  If not, why not?

We were only talking about state changes that violate the terms of the
contract between client and supplier (although Richard and I do not agree on
the exact nature of that contract).

Your example illustrates the point I was trying to make: the postcondition
is "formal.ref1.all = old formal.ref1.all + 1", and the subprogram does
indeed satisfy that postcondition.  Here, the state is completely public,
but no illicit state changes have taken place other that what were promised.

I suppose if you wanted to be completely doctrinaire you could pass Formal
as an in-out mode parameter, but I don't recommend this.  It's no secret
that Formal is a record the contains access objects, and that the function
inc_formal_ref1_object modifies the value designated by the access object,
not the access object itself.  So just pass Formal as in-mode.

Your function does exactly what it promised to do: nothing more, and nothing
less.  So as far as I am concerned, there is no problem with it.

--
Those who believe in the supernatural should be required to learn computer
programming.  This would force them to discover that things which appear to
be completely mysterious and incomprehensible, in fact have a logical, and
usually simple, explanation.

J. B. R. Yant, Mortal Words