Well, the array indices are allowed to be any sorts of integers, and the standard is worded in such a way that such addition must work correctly without overflowing — provided, of course, that the array you're indexing into is large enough. So in practice, you're bounded by the width of size_t.
Oh, and did you know that the width of ptrdiff_t is not required to have any relation to the width of size_t? There is even an explicit provision in standard for the case when subtracting two pointers into the same very large array can't fit into ptrdiff_t:
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; the result is the difference of the subscripts of the two array
elements. The size of the result is implementation-defined, and its type (a signed integer type) is
ptrdiff_t defined in the <stddef.h> header. If the result is not representable in an object of that
type, the behavior is undefined.
So yeah; you should try to work strictly with indices only, they're guaranteed to be correctly addible to pointers no matter the sizes of your arrays.
alt Hacker News
Well, the array indices are allowed to be any sorts of integers, and the standard is worded in such a way that such addition must work correctly without overflowing — provided, of course, that the array you're indexing into is large enough. So in practice, you're bounded by the width of size_t.
Oh, and did you know that the width of ptrdiff_t is not required to have any relation to the width of size_t? There is even an explicit provision in standard for the case when subtracting two pointers into the same very large array can't fit into ptrdiff_t:
So yeah; you should try to work strictly with indices only, they're guaranteed to be correctly addible to pointers no matter the sizes of your arrays.