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From: bobduff@world.std.com (Robert A Duff)
Subject: Re: Rules for Representation of Subtypes
Date: 1996/09/23
Message-ID: <Dy7917.2v2@world.std.com>#1/1
X-Deja-AN: 184853079
references: <mheaney-ya023080002209961353500001@news.ni.net>
 <01bba947$d8990620$188371a5@dhoossr.iquest.com>
organization: The World Public Access UNIX, Brookline, MA
newsgroups: comp.lang.ada
Date: 1996-09-23T00:00:00+00:00
List-Id: <comp.lang.ada>


In article <01bba947$d8990620$188371a5@dhoossr.iquest.com>,
David C. Hoos, Sr. <david.c.hoos.sr@ada95.com> wrote:
>My understanding of the rules for representation clauses for types is that
>the size attribute may only be specified for the first subtype, so for any
>subtype of integer you would get the same size as Integer.

No, that's not quite right.  You're right about the rules for rep
clauses.  But that certainly does *not* imply that all subtypes of
integer have the same size as Integer -- you can't *ask* the compiler to
use 8 bits for a subtype of Integer, but the compiler can use 8 bits.
In fact, 13.3(55) requires that they do *not* have the same size, in
general.

Besides, the A_Small_Integer in the example was a variable, not a
subtype.  You can specify its size as 8 bits, if you want.  Or 32 bits.

Also, it's good to keep in mind that if you have subtype S and object X
of subtype S, it is NOT always the case that S'Size = X'Size.  In fact,
it is unusual for these to be equal.

>In fact, I just tried to specify a size of 8 bits for your object
>"A_Small_Integer", with gnat 3.04a on Win95, and found that the compiler
>gave no warning, and ignored the clause.  Printing out the 'size attribute
>yielded 32.

GNAT is not the definition of the Ada language.  ;-)

>However, as far as your "UNIX" example is concerned (I say "UNIX", because
>this would apply to any OS where you're doing what you appear to be doing
>here -- i.e. interfacing to the C library), why not write the code in a
>manner where it doesn't matter how the compiler represents the object --
>i.e., write:
>
>    read (fd, A_Small_Integer'Address, A_Small_Integer'Size /
>System.Storage_Unit);

Yes, that's better than "4".

- Bob