// Factorial defined as a constexpr function definition.  (4ed page 793).
// A function that can be use by the compiler, as well as on run time.
// C++11

#include <iostream>

constexpr int fac(int i){
  return (i <= 0) ? 1 : i * fac(i - 1);
}

// compile-time test cases:
static_assert(fac(5)==120,      "fac(5)  problem");
static_assert(fac(10)==3628800, "fac(10) problem");

int main(){
  using namespace std;

  // CALL OF fac AT COMPILE TIME:
  constexpr int res = fac(10);
  cout << "fac(10) = " << res << endl;      // fac(10) = 3628800
  cout << "fac(5) = " << fac(5) << endl;    // fac(5) = 120

  // CALL OF fac AT RUN TIME:
  int i;
  cout << "Enter i: ";
  cin >> i;
  cout << "fac(2*i) " << fac(2*i) << endl;  // Depends in user input of i
}