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From: bobduff@world.std.com (Robert A Duff)
Subject: Re: Valid Attribute and Unchecked Conversion
Date: 1996/10/02
Message-ID: <Dynt6I.5D1@world.std.com>#1/1
X-Deja-AN: 186736896
references: <DyCF97.7zp@world.std.com> <1996Oct1.100608.1@eisner>
 <qw6buemy9ea.fsf@gargantua.enst.fr>
 <mheaney-ya023180000110962027510001@news.ni.net>
organization: The World Public Access UNIX, Brookline, MA
newsgroups: comp.lang.ada
Date: 1996-10-02T00:00:00+00:00
List-Id: <comp.lang.ada>


In article <mheaney-ya023180000110962027510001@news.ni.net>,
Matthew Heaney <mheaney@ni.net> wrote:
>So it's legal to check (using the valid attribute) the value of a (scalar)
         ^^^^^ you me "not erroneous".
>object with an invalid representation, right?

Right.

>Given the following program:
>
>   declare
>      type T is range 1 .. 10;
>      function To_T is new Unchecked_Conversion (Integer, T);
>      O : constant T := To_T (0);

I can't read that clearly on my screen, but I assume the variable name
is the letter oh, and the expression is the number zero.  ;-)

>   begin
>      if O'Valid then
>         ...
>   end;
>
>I want to know if the program is correct.  Is it erroneous, yes or no?

This is essentially the same as the example in 13.9.1(12.d), which I
posted a couple of days ago.  It is erroneous, by 13.9.1(12).  This has
nothing to do with the 'Valid attribute -- the above program becomes
erroneous before it gets to the 'Valid.  What I mean is, it's not that
'Valid doesn't work.  It's that Unchecked_Conversion is explicitly
defined to be erroneous in the above case.

Note that the rules for I/O are somewhat different than for
unchecked_conversion, primarily because function results are different
from 'out' parameters.

- Bob