// Factorial defined as a function template (4ed page 793).
// The input N is a templates parameter.
// C++11

#include <iostream>

template<int N>
constexpr int fac(){
  return N * fac<N-1>();
}

template<>                  // A specialization for N = 1 
constexpr int fac<1>(){
  return 1;
}


int main(){
  using namespace std;

  // call of fac at compile time:
  constexpr int res = fac<10>();
  static_assert(res==3628800, "Value of fac<10> is incorrect");
 
  cout << "fac(10) = " << res << endl;        // fac(10) = 3628800
  cout << "fac(5) = " << fac<5>() << endl;    // fac(5) = 120

  // attempt to call of fac at run time - not possible, however:
  int i;
  cout << "Enter i: ";
  cin >> i;
  cout << "fac(2*i) " << fac<2*i>() << endl;  // Compile time error: 
                                              // the value of 'i' is not usable in a constant expression
}