507: Mainline

At last, you have the following definitions for the box_car and tank_car classes:

class box_car : public railroad_car, public box { 
  public: // Default constructor: 
          box_car ( ) : box (10.5, 9.5, 40.0) { } 
          // Displayers: 
          virtual void display_short_name ( ) {cout << "box";} 
          virtual void display_capacity ( ) {cout << volume ( );} 
}; 
class tank_car : public railroad_car, public cylinder { 
  public: // Default constructor: 
          tank_car ( ) : cylinder (3.5, 40.0) { } 
          // Displayers: 
          virtual void display_short_name ( ) {cout << "tnk";} 
          virtual void display_capacity ( ) {cout << volume ( );} 
}; 

The volume function mentioned in the definition of the box_car display_capacity function must be the volume function inherited from the box class. Similarly, the volume function mentioned in the tank_car display_capacity function must be the volume function inherited from the cylinder class.