Overloading the output operator in c++

#include <iostream>
using namespace std;

class x
{
    int i;
public:
    x(int k)
    {
        i=k;
    }
    friend ostream& operator<<(ostream& os, const x& d1);
};

ostream& operator<<(ostream& os, const x& d1)
{
    os << d1.i ;
    return os;
}

int main()
{
    x s1(20);
    cout << s1;
}