Friend Function test

#include <iostream>
using namespace std;
class B;
class A
{
private:
    int hight;
public:
    A();
    friend int sum(A,B);
};
class B
{
private:
    int hight2;
public:
    B();
    friend int sum(A,B);
};
A::A()
{
    hight = 10;
}
B::B()
{
    hight2 = 20;
}
int sum (A a, B b)
{
    return (a.hight+b.hight2);
}

int main ()
{
    A a;
    B b;

    cout << sum(a,b);

    return 0;
}