定义一boat与car两个类，二者都有weight属性，定义二者的一个友元函数totalweight()，计算二者的重量和。
测试程序样例：
在这里给出函数被调用进行测试的例子。例如：
#include <iostream>
using namespace std;
/* 请在这里填写答案 */
int main()
{
int c,b;
cin>>c>>b;
car c1(c);
boat b1(b);
cout<<totalweight(b1,c1)<<endl;
}
输入样例：
1000 2000
输出样例：
3000
代码实现：
#include <iostream>
using namespace std;
class car;
class boat;
class car
{
int weight;
public:
car(int c)
{
weight = c;
}
friend boat;
friend int totalweight(boat &b, car &c);
};
class boat
{
int weight;
public:
boat(int b)
{
weight = b;
}
friend car;
friend int totalweight(boat &b, car &c);
};
int totalweight(boat &b, car &c)
{
return ( b.weight + c.weight );
}
