Re: private types

[Robert A Duff <bobduff@shell01.TheWorld.com>]

Justin Gombos <rpbkbq.xax.gld@uluv.kbq> writes:

> On 2006-03-18, Robert A Duff <bobduff@shell01.TheWorld.com> wrote:
> >
> > In Ada, if an object of an access type has no explicit initial
> > value, you can't easily tell whether that means "null is a
> > meaningful value for this variable, and that's the default I want"
> > versus "this variable will be initialized to a meaningful (non-null)
> > value later".
> 
> If you want to distinguish between the two possiblities, you could
> explicitly initialize your pointers to null in the first case, and not
> in the second.

Yes, that's a reasonable style, and I use it myself.
But there's no compiler support for that style.
As far as the language and compiler are concerned,
"X: T" and "X: T := null" mean EXACTLY the same thing.
It's hard to tell whether the programmer means the same thing
in these cases.

>...  I rarely use access types,

So you rarely create trees, linked list, etc?

>... and I probably wouldn't do
> that myself simply because I find the distinction unimportant for
> access types.  Regardless, I'm not going to give up the benefit of
> having this distinction on non-access scalars simply because my access
> type declarations don't have it.
> 
> > This is exactly analogous to the case with integers -- if they were
> > default-initialized to zero, you can't easily tell whether zero is
> > intended as a meaningful initial value, versus later initialization
> > to a meaningful value.
> 
> Integers, and other non-access scalars are different in this case
> because you cannot expect zero to have the same meaning.  Zero has a
> universal meaning with access types,

No, zero has no meaning with access types!

>... but it could be in range or out
> of range for any other type.  The ARM selects access types
> specifically to get a default initialization of zero for this reason.

There is no "default initialization of zero".  There is a default-init
to null, which has nothing to do with zero (according to the Ada
standard).

- Bob