An operator is a symbol that tells the complier to perform specifc mathematical or logical manipulations.

C++ allows u to specify more than one definition for an operator in the same scope, which is called operator overloading.

almost all operators can be overloaded in c++; except below:

1. socpe operator ::

2. sizeof 

3. member selector .

4. member pointer selector *

5. ternary operator ?:

我们期待C3 直接 变成7，9; 但是实际上编译的时候会报错， 因为对于class的对象而言还没有定义+这样的操作

如何定义呢：

这个例子中real 是C1 的；c.real是C2的；

返回一个临时的赋值给C3