这个重载（operator<<)为什么不行？

重新学习C++中。
遇到了问题：
重载<<时总编译有错：

/*

 * C plus plus learning.

 * Question: How to overloading << for a class/object.

 */

#include <iostream>

using namespace std;



class Point {

private:

    int x;

    int y;

public:

    Point();

    Point( int x, int y );

    ~Point();

    // FIXME: this overloading does not work

    ostream &operator<<(ostream &output, const Point &thiz);

};



Point::Point() {}

Point::Point(int ax, int ay) {

    x = ax;

    y = ay;

}

Point::~Point() {}



ostream &Point::operator<<(ostream &output, const Point &thiz) {

    cout << "(" << thiz.x << ", " << thiz.y << ")" << endl;

}



int main() {

    Point a( 3, 4 );

    cout << a;

    return 0;

}

以上代码编译有错：
overload.cpp:17: error: ‘std::ostream& Point::operator<<(std::ostream&, const Point&)’ must take exactly one argument
overload.cpp:27: error: ‘std::ostream& Point::operator<<(std::ostream&, const Point&)’ must take exactly one argument
不知道是为什么，哪位大侠能帮忙分析下？