18. Access control example

• 
  // point.h
  class Point
  {
  private:
      int m_x;
      int m_y;
      void reset_coord();
  public:
      double distance_to_origin();
  };
  
  // point.cpp
  #include <cmath>
  double Point::distance_to_origin()
  {
      // Access to private members m_x, m_y is allowed,
      // because we are in a member function:
      return sqrt( ( double ) m_x * m_x + m_y * m_y );
  }
  
  // main.cpp
  int main() // free function
  {
      Point pt;
      pt.m_x = 4;       // Error, m_x is private
      pt.reset_coord(); // Error, reset_coordinates() is private
      double dbl = pt.distance_to_origin(); // Ok, public
      return 0;
  }