// Ex6_01.cpp
  // Exercising pointers to functions
  include <iostream>
  using std::cout;
  using std::endl;
  
  long sum(long a, long b);              // Function prototype
  long product(long a, long b);          // Function prototype
  
  int main(void)
  {
    long (*pdo_it)(long, long);          // Pointer to function declaration
  
    pdo_it = product;
    cout << endl
         << "3*5 = " << pdo_it(3, 5);    // Call product thru a pointer
  
    pdo_it = sum;                        // Reassign pointer to sum()
    cout << endl
         << "3*(4 + 5) + 6 = " 
         << pdo_it(product(3, pdo_it(4, 5)), 6);   // Call thru a pointer,
                                                   // twice
    cout << endl;
    return 0;
  }
  
  // Function to multiply two values
  long product(long a, long b)
  {
    return a*b;
  }
  
  // Function to add two values
  long sum(long a, long b)
  {
    return a + b;
  }