第四章 面向对象编程
Object-oriented Programming

1. 类，构造函数和析构函数，类的指针
   Classes. Constructors and Destructors. Pointers to classes.
2. 操作符重载,this,静态成员
   Overloading Operators. this. Static members
3. 类之间的关系
   Relationships between classes: friend. Inheritance
4. 虚拟成员，抽象，多态
   Virtual Members. Abstraction. Polymorphism