Is D's pointer subtraction more permissive than C (and C++)?

[H. S. Teoh]

On Fri, Apr 01, 2022 at 01:56:19PM -0700, Ali Çehreli via Digitalmars-d wrote:
> On 4/1/22 13:47, Steven Schveighoffer wrote:
> 
> >> I think this rule is related to C's accepting wildly different
> >> platforms, some of which may have different kinds of memory. Two
> >> pointers to different kinds of memory may not be subtracted.
> >
> > Well, can the pointers be subtracted? Yes.
> 
> My understanding is that depending on the CPU, certain operations
> would make the CPU barf. I am not sure but the old protected memory,
> extended memory, etc. systems might not be able to subtract between
> the systems at all. (Not sure; I am making this up.)
[...]

In the bad ole days of segmented protected memory (around the days of
the 386 or 486, IIRC), you could have memory with different segment
prefixes, referenced using a convoluted scheme of far ptrs and near
ptrs. Near ptrs are relative to a particular segment; subtracting near
ptrs associated with diverse segment pointers yields nonsensical values.
You can subtract far ptrs, sorta-kinda, but the results are likely to be
either garbage, or else refer to an address that can't be addressed with
existing segment pointers. So basically, it's Trouble with a capital T.

On modern machines, though, this is no longer relevant. I think people
figured out real quick that segmented addressing is just way more
trouble than it's worth, so we came running back, tail between legs, to
the flat memory model and embraced it like there's no tomorrow. :-D


T

-- 
What doesn't kill me makes me stranger.