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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 10:51:37 -0400
Organization: The World Public Access UNIX, Brookline, MA
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Date: 2011-07-20T10:51:37-04:00
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okellogg <okellogg@users.sourceforge.net> writes:

> Picking up an age old thread,
> http://groups.google.com/group/comp.lang.ada/browse_thread/thread/9ab165cd2cc73cb

Wow.   1998.

> 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.

> Reusing the example from the OP,
>
>>    type x is record
>>      one : boolean;
>>      two : boolean;
>>      three : boolean;
>>    end record;
>>    for x use record
>>      one   at 0 range 0..0;
>>      two   at 0 range 3..3;
>>      three at 0 range 15..15;
>>    end record;
>
> What I imagine is something like,
>
> for x use record
>    one   at 0 range 0 .. 0;
>    null   at 0 range 1 .. 2;    // note reserved word "null"
>    two   at 0 range 3 .. 3;
>    null   at 0 range 4 .. 14;
>    three at 0 range 15 .. 15;
> end record;
> for x'Size use 16;
>
> The components with "null" would instruct the compiler to fill the
> bits with 0.

How about the following?

package P is
   type Bits2 is mod 2**2;
   pragma Assert (Bits2'Size = 2);
   type Bits11 is mod 2**11;
   pragma Assert (Bits11'Size = 11);

   type T is
      record
         One : Boolean;
         Gap1 : Bits2; -- must be zero
         Tooth : Boolean;
         Gap2 : Bits11; -- must be zero
         Ree : Boolean;
      end record
        with Predicate => T.Gap1 = 0 and T.Gap2 = 0;

   for T use
      record
         pragma Complete_Representation;
         One at 0 range 0..0;
         Gap1 at 0 range 1..2;
         Tooth at 0 range 3..3;
         Gap2 at 0 range 4..14;
         Ree at 0 range 15..15;
      end record;
end P;

with Ada.Assertions; use Ada.Assertions;
procedure P.Main is
   X : T := (One | Tooth | Ree => True, Gap1 => 0, Gap2 => 0); -- OK
begin
   begin
      X := T'(One | Tooth | Ree => True, Gap1 => 1, Gap2 => 2); -- Wrong.
   exception
      when Assertion_Error => null; -- OK
   end;
   pragma Assert (X = (One | Tooth | Ree => True, Gap1 => 0, Gap2 => 0));
end P.Main;

By the way, I find Ada's representation clauses to be at the wrong
level of abstraction.  Why can't I just write a single line of code
that means "put all the components in declaration order with no gaps
in between"?

- Bob