3、　　友元

编写C++程序完成以下功能：

（1）　　定义一个Boat和Car两个类，他们都具有私用属性——重量；

（2）　　编写一个函数，计算两者的重量和。

double TotalWeight(Boat& b, Car& c)

-3, friend write C++ program to accomplish the following functions: (1) defines two classes of a Boat and Car, they have private property- weight　(2) write a function to calculate both the weight and. double TotalWeight (Boat &　b, Car &　c)