类型兼容原则:

//1、引用就是对象或者变量的一个别名，它不是独立存在的。

//2、引用在定义的时候就必须初始化

//3、可以给引用赋值



//1、派生类的对象可以初始化基类的引用

//2、引用调用其类成员的时候，执行的是引用类方法。-Type compatible with the principle:// 1, the reference is an alias for an object or variable, it is not independent.// 2, when referenced in the definition must be initialized// 3, you can assign to a reference// 1, derived class object can initialize the base class reference// 2, the reference to call its class members when is the reference implementation of class methods.