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

[Steven Schveighoffer]

On 4/2/22 1:08 PM, Paul Backus wrote:
> On Saturday, 2 April 2022 at 02:20:34 UTC, Steven Schveighoffer wrote:
>> Right, but my larger point was, the *subtraction itself* is not harmful.
>>
>> There's two ways to look at this. First, if you subtract two pointers 
>> that *aren't to the same block*, then the data is garbage. The other 
>> way is that it is *undefined behavior*. I think *using* that 
>> subtracted difference to e.g. index a pointer is what would be UB. But 
>> the subtraction itself is ok.
> 
> It's UB just to perform the subtraction:
> 
>> [C11 § 6.5.6 ¶ 9][1] When two pointers are subtracted, both **shall** 
>> point to elements of the same array object, or one past the last 
>> element of the array object
> 
>> [C11 § 4 ¶ 2][2] If a ''shall'' or ''shall not'' requirement that 
>> appears outside of a constraint or runtime- constraint is violated, 
>> the behavior is undefined.
> 
> [1]: http://port70.net/~nsz/c/c11/n1570.html#6.5.6p9
> [2]: http://port70.net/~nsz/c/c11/n1570.html#4p2

Yep. That's a steep penalty.

It looks like C is trying to avoid having to specify how memory works 
without actually specifying it (likely on purpose so it doesn't tie down 
hardware developers to one memory model).

An interesting read I found on C and pointer comparisons here:

https://stefansf.de/post/pointers-are-more-abstract-than-you-might-expect/

-Steve