ØThe specialization relationship is typically implemented using inheritance.
Polymorphism(多态): There are two powerful aspects to inheritance.
ØOne is code reuse.
ØThe second is polymorphism(many-forms).
在父类中定义的虚函数，可以在子类中被重写。override是必要的。
Limitations of Abstract
ØAbstract does force all the derived classes to implement the method, but nothing forces the derived derived classes to implement its own DrawWindow() method.
