class CarDecoratorProgressTracker : public CarDecorator
{
  bool mHaveDoors;
  bool mHaveEngine;
  bool mHaveWheels;
...
  virtual void addEngine(Engine* engine) 
  {
    if (engine)
      mHaveEngine = true;
    CarDecorator::addEngine(engine);
  }
  virtual void addDoors(Door* doors, int doorsCount) 
  {
    if (doorsCount && doors)
      mHaveDoors = true;
    CarDecorator::addDoors(doors, doorsCount);
  }
...

  virtual bool readyToMove()
  {
    return (mHaveDoors && mHaveEngine && mHaveWheels);
  };
};