From mboxrd@z Thu Jan  1 00:00:00 1970
X-Spam-Checker-Version: SpamAssassin 3.4.4 (2020-01-24) on polar.synack.me
X-Spam-Level: 
X-Spam-Status: No, score=-1.3 required=5.0 tests=BAYES_00,INVALID_MSGID
	autolearn=no autolearn_force=no version=3.4.4
X-Google-Language: ENGLISH,ASCII-7-bit
X-Google-Thread: 103376,45a9122ddf5fcf5
X-Google-Attributes: gid103376,public
From: Ken Garlington <garlingtonke@lmtas.lmco.com>
Subject: Re: Rules for Representation of Subtypes
Date: 1996/10/01
Message-ID: <32515744.3B83@lmtas.lmco.com>#1/1
X-Deja-AN: 186558385
references: <DyCF97.7zp@world.std.com> <1996Sep26.191257.1@eisner>
 <DyEr6p.38H@world.std.com> <52hnvh$li0@wdl1.wdl.lmco.com>
 <DyKC85.Gqx@world.std.com> <1996Oct1.100608.1@eisner>
 <qw6buemy9ea.fsf@gargantua.enst.fr>
content-type: text/plain; charset=us-ascii
organization: Lockheed Martin Tactical Aircraft Systems
mime-version: 1.0
newsgroups: comp.lang.ada
x-mailer: Mozilla 2.02 (Macintosh; I; 68K)
Date: 1996-10-01T00:00:00+00:00
List-Id: <comp.lang.ada>


Samuel Tardieu wrote:
> 
> >>>>> "Larry" == Larry Kilgallen <kilgallen@eisner.decus.org> writes:
> 
> Larry> So can someone give an example of how Z'Valid _might_ be useful
> Larry> after an unchecked conversion, as seems to be indicated by the
> Larry> Reference Manual.
> 
> I don't think that the Valid attribute has been put in the language to
> be used after an Unchecked_Conversion, but rather to be used after
> calls to imported subprograms and calls to Read attributes (data
> coming from a stream).

I don't know much, but I do know this: I specifically wrote a revision
request to Ada 83 to handle the following situation:

  type Foo is range 1 .. 10;
  X: Foo; -- assume that this takes up a storage unit with a range
          -- greater than 1 .. 10
  ...

  Read(X); -- some I/O operation, unchecked conversion, whatever
           -- that might put a bit pattern in X that is not in 1 .. 10

  if X in Foo then
    raise Some_Exception; -- rarely works in Ada 83 due to optimization
  end if;

I thought one of the important uses of X'Valid was to be able to test 
for a legal bit pattern, and never mind the optimizations. If it doesn't 
work, then why did I go to all the trouble to write the RR?

-- 
LMTAS - "Our Brand Means Quality"