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, 计算二者的重量之和

#include <iostream>
using namespace std;
class car;
class boat
{
    public:

	    boat(int weight)
        {
            this->weight = weight;
        }

	    friend int getTotalWeight(boat &b,car &c);
    
    private:

	    int weight;
};

class car
{
    public:

	    car(int weight)
        {
            this->weight = weight;
        }

	    friend int getTotalWeight(boat &b,car &c);
    
    private:

	    int weight;
};

int getTotalWeight(boat &b,car &c)
{
	return b.weight+c.weight;
}

int  main()
{
	boat b(3);
	car c(2);

	cout<<"总重量是"<<getTotalWeight(b,c)<<endl;

    return 0;
}