C++自定义类重载运算符出现的一堆问题

今天我在自定义类重载运算符<<时遇到了一个很奇怪的报错......

先上最后能够正常运行的代码吧（相关头文件在stdafx.h里面，懒得打了）

// point.h

class point
{
public:
	point(int x, int y);

	string printPos(); // "[3,5]"
	friend ostream &operator<<(ostream &os, const point &p); // [3,5]

private:
	int X, Y;
};

// point.cpp

#include "stdafx.h"
#include "point.h"

point::point(int x, int y)
{
	X = x;
	Y = y;
}

string point::printPos() // "[3,5]"
{
	return "[" + to_string(X) + "," + to_string(Y) + "]";
}


ostream &operator<<(ostream &os, const point &p) // [3,5]
{
	os << "[" + to_string(p.X) + "," + to_string(p.Y) + "]" << endl;
	return os;
}

// ConsoleApplication2.cpp

#include "stdafx.h"
#include "point.h"

int main()
{
	point p(3, 5);
	cout << p;
    return 0;
}