Re: Size of 0..255 is not 8 bits?

[Markus Kuhn <Markus.Kuhn@cl.cam.ac.uk>]

Matthew Heaney wrote:
> Better is to throw a size clause on your object:
>
> type Value is range 0..255;
> V : Value;
> for V'Size use 8;

This did not change anything, V is still a 16-bit integer. I get
no difference whether the 'Size clause is for the type, the object,
both or none at all. Only

  type Value is mod 2**8;

gave me really an 8-bit unsigned integer variable.

> Throw a size clause on your object V, and see what that does.

Nothing.

I see, why 0..255 requires an at least  9-bit signed type, but
I am somewhat disappointed that neither the type'Size nor the
object'Size clause gives me a compiler warning that this is not
an 8-bit type.

So all this IMHO means that either gnat-3.10p contains an error
here or that John Barnes wrote nonsense in "Programming in Ada95",
1996, p. 526, where he writes "Thus

  type Byte is range 0..255;
  for Byte'Size use 8;

indicates that objects of the type Byte should occupy only 8 bits."

The weirdest effect is that if I read the file content "ABCDEFGH...",
I get only every second byte (65 67 69 ...) with Value'Read,
and not as I would have expected a sequence of 16-bit values
(65+66*2**8 67+68*2**8 ...) or constraint errors.

Any guru insight?

Other question: If the rest of a stream is too short for a
T'Read to succeed, what is supposed to happen? Constraint error?
The ARM doesn't say much on this subject.

Markus

-- 
Markus G. Kuhn, Security Group, Computer Lab, Cambridge University, UK
email: mkuhn at acm.org,  home page: <http://www.cl.cam.ac.uk/~mgk25/>