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C2679 binary '<<': no operator found which takes a right-hand operand of type 'const Position2D'

위의 에러가 떠서 해결이 되지 않습니다.

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컴파일러 버전 탓인지.. VS2015 쓰고있습니다. 

// Monster.h

#pragma once
#include "Position2D.h"


class Monster
{

private:
	std::string m_name; 
	Position2D m_location; 
public:
	Monster(const std::string name_in, const Position2D & pos_in)
		: m_name(name_in), m_location(pos_in)
	{}

	void moveTo(const Position2D & pos_target)
	{
		m_location.set(pos_target);
	}

	friend std::ostream & operator  << (std::ostream & out, const Monster & monster)
	{
		out << monster.m_name << "  " << monster.m_location; // m_,location으로 변경 깔끔해짐
		return out;
	}

};


//Position2D.h
#pragma once
#include <iostream>
#include <string>

class Position2D
{
private:
	int m_x;
	int m_y;

public:
	Position2D(const int & x_in, const int & y_in)
		:m_x(x_in), m_y(y_in)
	{}

	void set(const Position2D & pos_target)
	{
		set(pos_target.m_x, pos_target.m_y); 
	}
	void set(const int & x_target, const int & y_target)
	{
		m_x = x_target;
		m_y = y_target;
	}

	friend std::ostream & operator << (std::ostream & out, Position2D & pos2d)
	{
		out << pos2d.m_x << "  " << pos2d.m_y << std::endl;
		return out;
	}

};


// Monster.cpp


#include "Monster.h"

using namespace std;

int main()
{

	Monster mon1("jack", Position2D(0, 0));

	{
		mon1.moveTo(Position2D(1, 4));
		cout << mon1 << endl;
	}

	return 0;

}