Re: S'Write and How To Count Bytes

[Marin David Condic <mcondic.nospam@acm.org>]

tmoran@bix.com wrote:

> >... I will get out what I expect.
>   type Temperature is new Integer range 60 .. 80;
>   for Temperature'size use 8; -- or 7
> Is a Temperature'Write going to produce one byte, because that's how
> big Temperature is, or 4 bytes, because that's how big Integer is?
>   type R is record
>     Speed : Long_Integer;
>   end record;
>   for R use record
>     S at 0 range 0 .. 7;
>   end record;
> Will an R'Write produce one byte for S, or 8 or what?
>

Didn't say you couldn't create cases that would be tough to make work
consistently. Said that if I used my head and declared my own primitive
types that I would likely get what I wanted. It isn't that often that
someone can't manage to live with "for X'Size use 8"

>
> >A perfect solution would be controlled representation of the (tagged)
> >record types and a fast overlay of a Stream_Element_Array.
>   I remember this discussion, but not the bottom line.  What's wrong
> with
>   TAG_SIZE : constant := 4;
>   ...
>   type This_Buffer is new Buffer with record
>     Speed : Integer;
>     T : Temperature;
>   end record;
>   for This_Buffer use record
>     Speed at TAG_SIZE range 0 .. 7;
>     T at Tag_Size+1 range 0 .. 7;
>   end record;
> and then overlay a Stream_Element_Array?  (Or a
> System.Storage_Elements.Storage_Array?)
>   I'd rather trust to TAG_SIZE being 4, or at least a single thing to
> change, as I move between compilers, rather than trusting Stream stuff.

Oh, you get part way there with that. There's a slew of other problems that
come up and I'd just as soon not start that whole mess again. A lot of it
involves being able to get a representation clause on the record elements
and have the compiler give you what you want instead of what it wants.
Problems with freezing rules. Problems with alignments when you extend the
type, and so on. It really would be the elegant solution, but there doesn't
seem to be a good way to make it happen in such a way that Ada will
guarantee a compiler must do what you want it to do. I got tired of
fighting the compiler, so I put it on the shelf until a later language
revision. :-)

If you're really curious, we could discuss it off-line.

MDC
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Marin David Condic - Quadrus Corporation - http://www.quadruscorp.com/
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