Re: Ada Pointer Size Problem

[Dave Thompson <david.thompson1@worldnet.att.net>]

On Mon, 11 Oct 2004 05:18:58 GMT, Jeffrey Carter <spam@spam.com>
wrote:

> skidmarks wrote:
<snip: access types versus addresses>
> > Any idea of what I'm really missing?
> 
> Probably the difference between pointers in simple languages and access 
> values in high-level languages. In C/++, a pointer is an int is an 
> address. In Ada, they're 3 different things.

This is overstating the case.

In C a pointer value is in practice just an address. This isn't
formally required, but traditionally it's so; the standard specifies
(intentionally) semantics that allow it; and anything else is
unacceptably (to most users) inefficient, since C requires many array
accesses -- thus including character strings -- to go through
pointers, and all pointers of a given type to be the same -- unlike
non-all access or PL/I OFFSET.

But (unlike its predecessor B) a C pointer/address is not just an int.
It has addition and subtraction, but scaled by the target size
(stride) -- and standardly only required to work within a single
variables; there is not required to be a flat address space, although
that is common and some C program/mers unportably rely on it. Other
operations which would be meaningless like multiply and divide are
prevented, and conversions are explicit, like System.Address.

In C++ the same is true for builtin/standard pointers, but the
standard library provides a number of enhanced pointer-like types that
can be used transparently, and you (or someone) can write your own.

- David.Thompson1 at worldnet.att.net