#include <cstddef> // size_t, ptrdiff_t using Size = ptrdiff_t; template< class Item, size_t n > constexpr auto n_items( Item (&)[n] ) noexcept -> Size { return n; } #include <iostream> using namespace std; auto main() -> int { int const a[] = {3, 1, 4, 1, 5, 9, 2, 6, 5, 4}; Size const n = n_items( a ); int b[n] = {}; // An array of the same size as a. (void) b; cout << "Size = " << n << "\n"; }