Re: Ada 83 - avoiding unchecked conversions.

[Chris Brand <cbrand@ccgate.hac.com>]

Matthew Heaney wrote:
>
[cut]
> But knowing the representation of the data is only important at the
> hardware level, ie "the interface boundary."  Internal to the software
> there's a lot going on that doesn't depend on representation of the data,
> so we want to practice some information hiding by letting the compiler
> choose the representation.
> 
[cut]
> 
> This confusion is also the reason why shops often say "Thou shalt not use
> predefined types, because it's not portable."  This too is another silly
> rule.
> 
> It would be perfectly valid for such a rule at the interface layer, because
> I have to know the representation of my types.  But internal to the
> software system, I don't care, and using predefined types is just another
> form of information hiding.
> 
[examples cut]

I worked on a project where the rule was not to use predefined types.
We had a package defining the "base types" to use, with all their
representation clauses and everything and we had to base our types on
these.
The main type was an "Integer_16".

This was great while we were using a 286 target. It was only when we
ported
it to a 486 target (where 16-bit values need more work than 32-bit ones)
that we noticed the disadvantage of such a rule. Left to itself, the
compiler would have chosen a 16-bit value for most things on the 286 and
a 32-bit value on the 486.

Of course, if you take these things away, people have to think about
"what is a sensible upper bound for this type ?" :-)

-- 
Chris
Stating my own opinions, not those of my company.