了解为何使用抽象类，学习通过继承实现代码重用的机制和方法 掌握如何声明函数为纯虚函数 掌握如何利用纯虚函数，编写派生类的覆盖函数(Understand why you use abstract classes and learn how and how to implement code reuse by inheritance Master how to declare a function as a pure virtual function Master how to use th