Talk:cpp/types/ptrdiff t

Article change proposition (I'm not sure if it's correct, so, I will post it here for discussion instead of applying it myself):

The sentence:

"If an array is so large (greater than PTRDIFF_MAX elements, but less than SIZE_MAX bytes), that the difference between two pointers may not be representable as std::ptrdiff_t, the result of subtracting two such pointers is undefined."

Should it be:

If an array is so large (greater than PTRDIFF_MAX bytes, but less than SIZE_MAX bytes) ...

ptrdiff_t serves to keep result of subtracting a pointer from another one. The range of possible values resulting from such an operation depend on the length of the array in question, expressed in 'bytes'(char-s), not upon the length expressed in 'elements'. Thus, changing respective text to "greater than PTRDIFF_MAX bytes" would make overall statement correct. As for "but less than SIZE_MAX bytes" part, it points to the fact that no valid object is greater in size than SIZE_MAX bytes, so it seems correct if somewhat unnecessary. I'm not sure if I a'v been missing something out.

Cpprf acount (talk) 11:54, 3 November 2017 (PDT)

The result of subtracting two pointers is, in fact, the number of elements, not the number of bytes. The statement is correct as written. T. Canens (talk) 12:51, 3 November 2017 (PDT)

array length stored in std::ptrdiff_t

The following is wrong:

"For char arrays shorter than PTRDIFF_MAX, std::ptrdiff_t acts as the signed counterpart of std::size_t: it can store the size of the array of any type and is, on most platforms, synonymous with std::intptr_t."

Better is:

std::ptrdiff_t acts as the signed counterpart of std::size_t: it can store the size of the array of any type and is, on most platforms, synonymous with std::intptr_t."

And add:

Arrays have a maximum object size of PTRDIFF_MAX, meaning that the difference between the starting elment and the element one-past-the-last-possible-valid-element is 0-PTRDIFF_MAX (equal to PTRDIFF_MIN+1); or if the order is swapped the subtraction is PTRDIFF_MAX-0. Both are validly representable in a std::ptrdiff_t

PS: https://wandbox.org/permlink/a3o3KP7vKOUGdEzF

Notice that the standard specifically allows for the difference between two array indices to not be representable in a ptrdiff_t, and the maximum size of an object is simply representable in a size_t --Ybab321 (talk) 03:15, 13 March 2023 (PDT)