Macro definition ARRAY_SIZE

Question

I encountered the following macro definition when reading the globals.h in the Google V8 project.

// The expression ARRAY_SIZE(a) is a compile-time constant of type
// size_t which represents the number of elements of the given
// array. You should only use ARRAY_SIZE on statically allocated
// arrays.

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

My question is the latter part: static_cast<size_t>(!(sizeof(a) % sizeof(*(a))))). One thing in my mind is the following: Since the latter part will always evaluates to 1, which is of type size_t, the whole expression will be promoted to size_t.

If this assumption is correct, then there comes another question: since the return type of sizeof operator is size_t, why is such a promotion necessary? What's the benefit of defining a macro in this way?

Answer 1

If sizeof(a) / sizeof(*a) has some remainder (i.e. a is not an integral number of *a) then the expression would evaluate to 0 and the compiler would give you a division by zero error at compile time.

I can only assume the author of the macro was burned in the past by something that didn't pass that test.