关于友元函数的问题c＋＋编程

-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。

//点击在此输入一行或多行代码
class Boat
{

};
class Car
{

};

float getTotalWeight(Boat& b, Car& c)
{
return b.weight + c.weight;
}

int main()
{
// 点击在此输入代码
Boat boat; Car car;
cout << getTotalWeight(boat, car) << endl;
return 0;
}

#include <iostream>
using namespace std;
class car;
class boat{
private:
    int weight;
public:
    boat(int k){weight =k;}
    friend int gettotalweight(boat &b,car &c);
};
class car{
private:
    int weight;
public:
    car(int k){weight =k;}
    friend int gettotalweight(boat &b,car &c);
};
int gettotalweight(boat &b,car &c){
    return b.weight+c.weight;
}
void main(){
    boat b1(5);
    car c2(2);
    cout<<gettotalweight(b1,c2)<<endl;
}