Designing Game Code with Polymorphism - Learn C++ for Game Development - page 123

hierarchy as having the base class at the top and all child classes below that. Casting from a higher
class to a lower class is a downcast and the opposite is an upcast. The code in Listing 11-5 is safer
than the code in Listing 11-4, as we use dynamic_cast to upcast to the Vehicle pointer.
Listing 11-5. Upcasting
void PrintNumberOfWheels()
{
Vehicle vehicle;
Car car;
Motorcycle motorcycle;
Vehicle* pVehicle = dynamic_cast<Vehicle*>( & vehicle);
std::cout << pVehicle->GetNumberOfWheels() << std::endl;
pVehicle = dynamic_cast<Vehicle*>( & car);
Car* pCar = dynamic_cast<Car*>(pVehicle);
if (pCar != nullptr)
{
std::cout << pCar->GetNumberOfWheels() << std::endl;
}
pVehicle = dynamic_cast<Vehicle*>( & motorcycle);
pCar = dynamic_cast<Car*>(pVehicle);
if (pCar != nullptr)
{
std::cout << pCar->GetNumberOfWheels() << std::endl;
}
}
The upcasts to Vehicle in Listing 11-5 are all much safer than the previous implicit casts. This would
not matter too much in this example code, as we know all of the objects derive from Vehicle , but in
production code you might not be able to guarantee that a given pointer derives from a specific
base class.
Creating Interfaces with Pure Virtual Methods
The Vehicle class has a value to return for GetNumberOfWheels . In a proper program it would be
unlikely that we would want to be able to create basic Vehicle objects as we are more likely to want
to create instances of Car or Motorcycle . It also makes little sense for a Vehicle to have any number
of wheels, even none. We can turn Vehicle into an interface by making its GetNumberOfWheels class a
pure virtual method. Listing 11-6 shows how to do this.
Listing 11-6. Making Pure Virtual Methods
class Vehicle
{
public:
virtual unsigned int GetNumberOfWheels() const = 0;
};