Re: Should representation clauses be complete for each bit?

[okellogg <okellogg@users.sourceforge.net>]

On Jul 20, 7:27 pm, Robert A Duff <bobd...@shell01.TheWorld.com>
wrote:
> [...]
> 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.

Yes, I guess you're right.
The syntax you propose is much more general and flexible.

> 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;

I'd imagine that a default assignment at Gap1 and Gap2 would suffice?
         Gap1 : Bits2 := 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;
>

I believe the compiler would warn about incomplete representation at
T1?

Also I guess an upcast from T2 to T1 should do for converting the
types but I'm not so sure about the downcast from T1 to T2?

-- Oliver