友元的使用

#include<iostream>
class Boat;//需要声明，因友元用到
class Car
{

int size;
public:

void setSize(int j){size=j;}
int getSize(){return size;}
friend int leisure(int time,Car& aobj,Boat&bobj);//
};
class Boat
{
int size;
public:
void setSize(int j){size=j;}
int getSize(){return size;}
friend int leisure(int time,Car& aobj,Boat&bobj);//
};
int leisure(int time,Car& aobj,Boat&bobj)
{
return time*aobj.size*bobj.size;//
}
int main()
{
Car c1;
c1.setSize(2);
Boat b1;
b1.setSize(3);
std::cout<<leisure(5,c1,b1);
}