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From: Robert A Duff <bobduff@shell01.TheWorld.com>
Newsgroups: comp.lang.ada
Subject: Re: Should representation clauses be complete for each bit?
Date: Wed, 20 Jul 2011 13:27:58 -0400
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Date: 2011-07-20T13:27:58-04:00
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okellogg <okellogg@users.sourceforge.net> writes:

> On 20 Jul., 16:51, Robert A Duff <bobd...@shell01.TheWorld.com> wrote:
>> [...]
>> > IMHO it would be a real gain if we could explicitly mention the unused
>> > bits in the rep spec.
>>
>> I think the unused bits should be explicitly declared.
>
> My use case originates from a bit packed communication protocol where
> the standard requires spare bits interspersed among the useful data.
> However the API of the driver shall not expose these spare fields.

I see.

But that could happen with any component -- it doesn't have to be
"spare" and set to zero.  What if the protocol requires "set this
3-bit gap to all 1's"?  Or what if there's a checksum component,
which is set by the driver just before shipping the thing over
the wire -- in that case, you wouldn't want to expose the checksum
to clients.

Perhaps this is what you really want (note that this is illegal
for all sorts of reasons):

   type T1 is
      record
         One : Boolean;
         Tooth : Boolean;
         Ree : Boolean;
      end record;

   type T2 is new T1 with private; -- not Ada!
private
   type T2 is new T1 with
      record
         Gap1 : Bits2; -- must be zero
         Gap2 : Bits11; -- must be zero
      end record
        with Predicate => T.Gap1 = 0 and T.Gap2 = 0;

   for T1 use
      record
         pragma Complete_Representation;
         One at 0 range 0..0;
         Tooth at 0 range 3..3;
         Ree at 0 range 15..15;
      end record;

   for T2 use
      record
         pragma Complete_Representation;
         Gap1 at 0 range 1..2;
         Gap2 at 0 range 4..14;
      end record;

- Bob