446: Mainline

Definitions of the railroad_car class and that class's subclasses can contain all sorts of member variables, default constructors, constructors with arguments, and other member functions. For the moment, for the sake of simplicity, suppose that you strip out all that detail, leaving only default, do-nothing constructors:

class railroad_car { 
  public: railroad_car ( ) { } 
}; 
class box_car : public railroad_car { 
  public: box_car ( ) { } 
}; 
class tank_car : public railroad_car { 
  public: tank_car ( ) { } 
}; 
class engine : public railroad_car { 
  public: engine ( ) { } 
}; 
class caboose : public railroad_car { 
  public: caboose ( ) { } 
};