How to use a common Friend Function in c++?

Question

When i execute the code below it generates 2 errors c1::x is not accessible and missing ) in line9. Please explain. Thanks in advance...

#include<iostream>
class c1{
    int x;
    public:
    void input(){
        cout<<"Enter length : ";
        cin>>x;
    }
    friend void output(c1 obj1, c2 obj2);
};
class c2{
    int y;
    public:
    void input(){
        cout<<"Enter breadth : ";
        cin>>y;
    }
    friend void output(c1 obj1, c2 obj2);
};
void output(c1 obj1, c2 obj2){
    cout<<"Area is "<<obj1.x*obj2.y;
}
int main(){
    c1 obj1;
    c2 obj2;
    clrscr();
    obj1.input();
    obj2.input();
    output(obj1, obj2);
    getch();
    return 0;
}

Answer 1

The friend function needs to know that classes c1 and c2 exist. c1 is fine, because the friend is declared with in that class. But for c2 you need a forward declaration before the first friend declaration:

#include<iostream>

using std::cin;
using std::cout;

class c2; // forward declaration

class c1{

 ....
  friend void output(c1 obj1, c2 obj2); // OK now

};

I also added some missing declarations. You also need the headers for getch and clrscr.