定义Boat与car两个类，二者都有weight属性，定义二者的一个友元函数gettotalweight（），计算二者的重量和。

#include<iostream>

class Car;

class Boat

{

private:

     int weight ;

public:

 Boat(int);

 void getTotalWeight( Car &);

};

class Car

{   

 private:

     int weight ;

public:

 Car(int);

 friend void Boat :: getTotalWeight( Car &);  

};

Boat::Boat(int w)

{

 weight=w;

}

Car::Car(int w)

{

 weight=w;

}

void Boat :: getTotalWeight( Car &t)

{

 int sum=weight+t.weight;

    std::cout<<sum;

}

int main ()

{

 Boat b(2);

 Car c(3);

 b.getTotalWeight(c);

}