Number of array elements (for static sized arrays)

sizeof_array.cpp

#include <iostream>

// C-Style way of obtaining number of elements in 
// statically sized array.
// This is very unsafe!
#define SIZEOF_ARRAY(v) (sizeof(v)/sizeof(*v))

// C++-ish way for obtaining the number of array elements.
// Safe, cannot be compiled for simple ptr (array-like) types.
template<class T, std::size_t N>
constexpr decltype(N) sizeof_array(const T(&)[N]) noexcept
{
  return N;
}

int main()
{
  const char arr_c[] = "Das ist ein Test!!!";
  int arr_i[] = {1, 2, 3, 4, 5};
  double arr_d[5];

  const char *test = arr_c;

  std::cout << "sizeof(arr_c)=" << sizeof(arr_c)       // gives 20
    << ", sizeof_array(arr_c)=" << sizeof_array(arr_c) // gives 20
    << ", SIZEOF_ARRAY(arr_c)=" << SIZEOF_ARRAY(arr_c) // gives 20
    << std::endl

    << "sizeof(arr_i)=" << sizeof(arr_i)               // gives 20
    << ", sizeof_array(arr_i)=" << sizeof_array(arr_i) // gives 5
    << ", SIZEOF_ARRAY(arr_i)=" << SIZEOF_ARRAY(arr_i) // gives 5
    << std::endl

    << "sizeof(arr_d)=" << sizeof(arr_d)               // gives 40
    << ", sizeof_array(arr_d)=" << sizeof_array(arr_d) // gives 5
    << ", SIZEOF_ARRAY(arr_d)=" << SIZEOF_ARRAY(arr_d) // gives 5
    << std::endl

    << "sizeof(test)=" << sizeof(test)                 // gives 8 (not always expected)
    << ", sizeof_array(test)=" << sizeof_array(test)   // safe: won't compile for non static sized array types
    << ", SIZEOF_ARRAY(test)=" << SIZEOF_ARRAY(test)   // unsafe: gives 8 (!!!)
    << std::endl;
}