Is there a way for countof() to test if its argument is an array?

Question

A classic macro to compute the number of elements in an array is this:

#define countof(a)  (sizeof(a) / sizeof(*(a)))

The problem with this is it fails silently if the argument is a pointer instead of an array. Is there a portable way to ensure this macro is only used with an actual array by generating a compile time error if a is not an array?

EDIT: my question seems to be a duplicate of this one: Array-size macro that rejects pointers

Answer 1

Using a non-portable built-in function, here is a macro to perform a static assertion that a is an array:

#define assert_array(a) \
     (sizeof(char[1 - 2 * __builtin_types_compatible_p(typeof(a), typeof(&(a)[0]))]) - 1)

It works with both gcc and clang. I use it to make the countof() macro safer:

#define countof(a)  (sizeof(a) / sizeof(*(a)) + assert_array(a))

But I don't have a portable solution for this problem.