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From: Simon Wright <simon.j.wright@mac.com>
Newsgroups: comp.lang.ada
Subject: Re: Enumeration representation clause surprise.
Date: Thu, 12 Jun 2008 21:52:56 +0100
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Date: 2008-06-12T21:52:56+01:00
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Mike Silva <snarflemike@yahoo.com> writes:

> I wonder then if this is an error at all.  IANALL, but if the
> program makes available your specified values when queried with the
> proper mechanism (in this case, unchecked conversion), then I don't
> see what difference it makes how it stores the representations in
> raw memory.

The only reason I have ever written representation clauses is to
ensure that the contents of raw memory are what some external hardware
(or software, eg at the other end of a network connection) requires
them to be. So I really really don't want the compiler to do anything
other than what I asked for; and if it can't, to reject the
representation loudly and fatally!

Given OP's declarations

   type Enum_T is (One, Two);
   for Enum_T use (One => 1, Two => 2);

   type Record_T is record
      A1 : Enum_T;
      A2 : Enum_T;
   end record;
   for Record_T use record
      A1 at 0 range 0 .. 0;
      A2 at 1 range 0 .. 7;
   end record;
   for Record_T'Size use 2 * 8;

   Foo : Record_T;

there's a huge difference between an unchecked conversion of the value
Foo.A1 to <whatever>, which might quite reasonably have the bias
removed, and UC of the value Foo as a whole, which wouldn't.