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From: Robert Dewar <robert_dewar@my-deja.com>
Subject: Re: Newbie question : types , representation
Date: 1999/08/22
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Date: 1999-08-22T00:00:00+00:00
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In article <37BFC251.601ADF8F@village.uunet.be>,
  Jos De Laender <De_Laender-De_Winter@village.uunet.be> wrote:
> In C I could do it within 5 minutes.

Probably by making unwarranted assumptions :-)

> It would involve some bitmanipulation (shifting , anding ,
> oring ) on
> the character itself and then outputting. That's it.
>
> However, what's unclear is how to do this in ADA types : A
> character
> cannot be bit manipulated , and so needs some explicit
> translation to a
> 'byte' or something.
> How is this usually done ? I would guess to use a modular type
2**8 ? If
> I declare such a type , and I want a byte , is this guaranteed
by 2**8
> or do I have explicitly  to attribute it with a 'for Byte'size
use 8' ?

Is char guaranteed to be 8 bits in C, answer no ...

> Also I will need some kind of assignment to that modular type,
say aVar
> := 2#00000011#. This will assign the value 3 to aVar ? Is this
> guaranteed to have that bit pattern ?

nope, and of course there is no such guarantee in C either

> Or could I imagine a (probably
> exotic) machine on which 3 is represented with another bit
pattern ?

If you want to imagine such machines, your imagination is
equally relevant in C or Ada.

> Does ADA guarantee ?
> If I output aVar with a write function , is the bitpattern
guaranteed ?

nope, and neither does C

> Thanks for all clarifications.

Bottom line here: Don't get carried away by the formalism here.
Write reasonable code with reasonable assumptions, as you would
in C. Your low level bit twiddling stuff may be a target
dependent, at least from a theoretical point of view, but that's
not a terrible crime (and certainly no worse a crime in Ada
than in C :-)


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