多态描述的练习，掌握虚函数的定义方法和运行时多态的实现机制，从而理解虚函数的作用。 掌握纯虚函数的定义方法，理解抽象基类的作用-Polymorphic exercises described master virtual function definition and run-time polymorphism implementation mechanism, in order to understand the role of virtual functions. Grasp the d