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From: bobduff@world.std.com (Robert A Duff)
Subject: Re: Rules for Representation of Subtypes
Date: 1996/09/27
Message-ID: <DyEr6p.38H@world.std.com>#1/1
X-Deja-AN: 185696575
references: <mheaney-ya023080002209961353500001@news.ni.net>
 <mheaney-ya023080002409960726040001@news.ni.net> <DyCF97.7zp@world.std.com>
 <1996Sep26.191257.1@eisner>
organization: The World Public Access UNIX, Brookline, MA
newsgroups: comp.lang.ada
Date: 1996-09-27T00:00:00+00:00
List-Id: <comp.lang.ada>


In article <1996Sep26.191257.1@eisner>,
Larry Kilgallen <kilgallen@eisner.decus.org> wrote:
>Like:
>
>	if X'Valid
>
>?  I would think that as being an argument in _favor_ of declaring
>the C-updated object in a tightly constrained fashion.

No, that won't work.  By the time you get to that if statement, the
program execution is already erroneous.  Compilers can and do (and
should) take advantage of that fact to generate faster code that doesn't
work.  An unchecked conversion is a function call, and there's no way to
capture the result of it without assigning it somewhere, which causes
erroneousness if the result is bad.

- Bob