5-14

#include<iostream.h>
class car;
class boat
{
public:
 boat(double bw):bw(0){}
 void setbw(double );
 friend void totalweight(boat &a,car & b);
private:
 double bw;
};
class car
{
public: 
 car(double cw):cw(0){}
 void setcw(double );
 friend void totalweight(boat &a,car & b);
private:
 double cw;
};
void boat::setbw(double x)
{
 bw=x;
}

void car::setcw(double y)
{
 cw=y;
 
}
void totalweight(boat &a,car & b)
{
 cout<<"output the total weight of the boat and the car   "<<a.bw+b.cw<<endl;
}
void main()
{
 double cw,bw;
 boat b(20);
 car m(10);
 totalweight(b,m);
 cout<<"input the boat weight"<<endl;
 cin>>bw;
 b.setbw(bw);
 cout<<"input the car weight"<<endl;
 cin>>cw;
 m.setcw(cw);
 totalweight(b,m);
}

熟悉友元函数的使用。