运算符重载的问题

#include<iostream>
using namespace std;
class complex
{
private:
    double x,y;
public:
    complex(double a=0,double b=0)
        :x(a),y(b)
    {}
    friend ostream& operator <<(ostream& os,const complex& rhs);
};
ostream& operator <<(ostream& os,const complex& rhs)
{
    os<<rhs.x<<"   "<<rhs.y<<endl;
    return os;
}
int main()
{
    complex p(1,2);
    cout<<p<<endl;
}