18. Template specialization trick

• A cute template specialization trick: factorial calculation made at compile time:

  
      // factorial.cpp
      template < int N >
      struct Factorial
      {
          enum { value = N * Factorial< N-1 >::value };
      };
  
      template < >
      struct Factorial< 1 >
      {
          enum { value = 1 };
      };
  
      // main.cpp
      #include <iostream>
      int main()
      {
          const int fact5 = Factorial< 5 >::value;
          std::cout << fact5 << '\n';
          return 0;
      }
      // Program prints 120