For iterating though an array should we be using size_t or ptrdiff_t?

Question

In this blog entry by Andrey Karpov entitled, \"About size_t and ptrdiff_t\" he shows an example,

for (ptrdiff_t i = 0; i < n; i++)
  a[i] = 0;
         


        

Answer 1

In a blog post, I argue that you should always refrain from allocating memory blocks larger than PTRDIFF_MAX(*), because doing so will make compilers such as Clang and GCC generate nonsensical code even if you do not subtract pointers to that block in a way that causes the result to overflow.

(*) Even if malloc succeeds when you pass it a value larger than PTRDIFF_MAX. The crux of the problem is that GCC and Clang only generate code that behaves correctly when linked with such a malloc, but Glibc provides a malloc function that does not implement this limitation.

If you follow that constraint (which I encourage you to: that's the message of the blog post), then both types are equally correct.

This said, since only positive offsets need to be represented, size_t would be the natural choice in your example.