// CS2 Chung,
// - Default arguments (convenient for constructors)
// - Review Friend Function: not a member function, but can access private 
//   data of its parameters.
#include <iostream.h>
#include <math.h>
class Point
{  private:
      double x;
      double y;
   public:
      Point(double init_x = 0, double init_y = 0) // default arguments
      {  x = init_x;
         y = init_y;
      }
   /* Point()  
      {  cout << "Enter x and y for a point: ";
         cin >> x >> y;
      }                     Ambigous!!!!  
   */
      void display()
      {  cout << "(" << x << ", ";
         cout << y << ")" << endl;
      }
      friend Point middle(Point& p1, Point& p2);
};

// Point Point::middle(Point& p1, Point& p2) Wrong! not an mem. func.!
Point middle(Point& p1, Point& p2)
{
    double x_mid, y_mid;
    x_mid = (p1.x + p2.x) / 2;
    y_mid = (p1.y + p2.y) / 2;
    Point midpoint(x_mid, y_mid);
    return midpoint;
}

void main()
{
   Point p0, p1(1,2), p2(-1);

   middle(p1,p2);
   cout << "The midle point between two points: ";
   middle(p1,p2).display();
}