used of friend function in C++

/* perform addition of two number which is private for seprate class using friend function*/
#include<iostream>
using namespace std;
class B;//forward declaration
class A
{
    int x;
    public:
    A()//default cons
    {
        x=10;
    }
    friend void add(A,B);//declaration
};
class B
{
    int y;
    public:
    B()
    {
        y=20;
    }
    friend void add(A,B);//declaration
};
void add(A a1,B b1)
{
    cout<<"Sum of two diff class private member is "<<a1.x+b1.y;
}
int main()
{
    A a1;
    B b1;
    add(a1,b1);
}