warning 자꾸납니다.

소스는 다음과 같습니다.

#include <iostream>

using namespace std;

class Point
{
public :
		Point();
		Point( int ax, int ay );

		void SetPoint( int ax, int ay );

		friend ostream& operator << ( ostream& co, Point pt );

		friend Point& operator+( const Point& pt1, const Point& pt2 );
		friend Point& operator-( const Point& pt1, const Point& pt2 );

private :
		int x, y;

};

Point::Point()
{
	x = y = 0;
}

Point::Point( int ax, int ay )
{
	SetPoint( ax, ay );
}

void Point::SetPoint( int ax, int ay )
{
	x = ax;
	y = ay;
}

ostream& operator << ( ostream& co, Point pt )
{
	co << "(" << pt.x << "," << pt.y << ")";
	return co;
}

Point& operator+( const Point& pt1, const Point& pt2 )
{
	Point ptSum( pt1.x + pt2.x, pt1.y + pt2.y );
	return ptSum;
}

Point& operator-( const Point& pt1, const Point& pt2 )
{
	Point ptSub( pt1.x - pt2.x, pt1.y - pt2.y );
	return ptSub;
}

int main( void )
{
	Point pt1( 100, 100 ), pt2( 100, 200 );
	Point pt3, pt4;

	pt3 = pt1 + pt2;
	pt4 = pt1 - pt2;
	
	cout << pt1 << "+" << pt2 << "=" << pt3 << endl;
	cout << pt1 << "-" << pt2 << "=" << pt4 << endl;
	return 0;
}

다음과 같은 warning 납니다.

upoperator.cpp: In function `Point& operator+(const Point&, const Point&)':
upoperator.cpp:47: warning: reference to local variable `ptSum' returned
upoperator.cpp: In function `Point& operator-(const Point&, const Point&)':
upoperator.cpp:53: warning: reference to local variable `ptSub' returned