Organizing Game Objects - Learning C# by Programming Games - page 160

Something similar is happening in the Update method of the Ball class. At the end
of that method, we call the Update method of the ThreeColorGameObject class, so that
the position is changed according to the velocity. You can also see an example of
this in the new PaintCan class, which is also a subclass of ThreeColorGameObject .
10.5.4 Polymorphism
Because of the inheritance mechanism, there are now some interesting things hap-
pening. Since a cannon object is a three colored game object, this instruction is
allowed:
ThreeColorGameObject cannon = new Cannon(...);
Now suppose that we would then execute the following instruction:
cannon.Reset();
Which version of the Reset method would be called? The one in the Cannon class
or the one in the ThreeColorGameObject class? Although the variable cannon is of
type ThreeColorGameObject , it actually refers to an object of type Cannon . When you
run the program, the compiler maintains a so-called vtable which contains for each
object in the program to which inheritance tree it belongs and what that tree looks
like. Whenever a virtual method is called, like what happens here in the case of
the Reset method, the compiler looks in the table to find out which version of the
method should be called. In this case, it will see that we're dealing with a Cannon
object, so it will call the version of the Reset method that is defined in the Cannon
class.
This effect is called polymorphism and it comes in very handy sometimes. An-
other example where polymorphism is responsible for the behavior of the program
is when we call the Run method on the game variable. The Run method is defined
inside the Game class. This method does not know that it happens to work with a
Painter object. Because of polymorphism, when, for instance, the Update method is
called from within the Run method, automatically the right version of the method is
called, which would be the Update method that we defined in the Painter class.
Another example of where polymorphism is useful is when a game company
wants to release an extension of their game. For example, they might want to intro-
duce a few new enemies, or skills that a player can learn. They can provide these
extensions as subclasses of generic Enemy and Skill classes. The actual game engine
would then use these objects without having to know which particular skill or en-
emy it is dealing with. It simply calls the methods that were defined in the generic
classes.