c preprocessor - ARRAYSIZE C++ macro: how does it work?

Question

OK, I'm not entirely a newbie, but I cannot say I understand the following macro. The most confusing part is the division with value cast to size_t: what on earth does that accomplish? Especially, since I see a negation operator, which, as far as I know, might result in a zero value. Does not this mean that it can lead to a division-by-zero error? (By the way, the macro is correct and works beautifully.)

#define ARRAYSIZE(a) 
  ((sizeof(a) / sizeof(*(a))) / 
  static_cast<size_t>(!(sizeof(a) % sizeof(*(a)))))

Answer 1

The first part (sizeof(a) / sizeof(*(a))) is fairly straightforward; it's dividing the size of the entire array (assuming you pass the macro an object of array type, and not a pointer), by the size of the first element. This gives the number of elements in the array.

The second part is not so straightforward. I think the potential division-by-zero is intentional; it will lead to a compile-time error if, for whatever reason, the size of the array is not an integer multiple of one of its elements. In other words, it's some kind of compile-time sanity check.

However, I can't see under what circumstances this could occur... As people have suggested in comments below, it will catch some misuse (like using ARRAYSIZE() on a pointer). It won't catch all errors like this, though.