// composition.cc
  // we have covered inheritance, now we add composition
  // once students learn about inheritance, they tend to over use it.
  // it must be remembered that objects may be composed of other objects
  // composition has a different meaning and use than inheritance

  class door { // for a car door  each usually a header file, e.g. door.h
    public:
    private:
      float width, height, mass; // etc.
  };

  class engine { // for a car engine
    public:
    private:
      float horse_power, torque, mass; // etc.
  };

  class hood { // for a car hood
    public:
    private:
      float width, height, mass; // etc.
  };

  class wheel { // for a car wheel
    public:
    private:
      float diameter, width, mass; // etc.
  };

  class transporter { // for a physical transporter
    public:
      float get_mass(transporter object);
    private:
      float mass;
  };               // should be abstract

  // #include "transporter"  // all one file, so #include  commented out
  class vehicle : public transporter { // for a physical vehicle
                                       // inherits "is a" transporter
    public:
    private:
      float new_variable; // and more
  } vehicle_1, vehicle_2;              // can have objects

  //#include "vehicle.h"       // all in this file
  //#include "door.h"
  //#include "engine.h"
  //#include "hood.h"
  //#include "wheel.h"
  class car : public vehicle { // for physical car
                               // inherits "is a" vehicle
    public:
    private:
      door left_door;              // composition, car "has a" door
      door right_door;
      engine v8;                   // conposition, car "has a" engine
      hood my_hood;                // composition, car "has a" hood
      wheel lf,lr,rf,rr;           // composition, car has wheels
            // multiple inheritance will not work to get four wheels!
      // vehicle a_vehicle; WRONG!  by inheritance, car "is a" vehicle
  };

  int main()
  {
    car c1, c2; // c1 and c2 are two objects, instances of class car
  }