Type-safe way to get the length of an array in C++.

sizeof_array.hpp

// In C/C++, the idiom (sizeof(arr) / sizeof(*arr)) is often used to get the length of an array.
// But this will return incorrect results if arr is not actually an array, but a pointer.
// The following function template provides a type-safe way to get the length of an array,
// by "destructuring" the type and reading the array length from there.

#include <cstddef> // for size_t

template <typename T, size_t N>
constexpr size_t sizeof_array(T (&)[N])
{
  return N;
}

// here is a fallback for compilers without support for constexpr:
// this function takes a reference to array of type T[N], and returns a reference to char.
// the return type is then used with sizeof() to get N (compare with MSVC's _countof).
template <typename T, unsigned int N> char (&_return_sizeof_array(T (&)[N]))[N];

#define sizeof_array(__arr__) sizeof(_return_sizeof_array((__arr__)))