c++ - why should i return reference when i compare three objects? -

i tring learn operator overloading , dont somthing. when do:

    class point     { public:     void show(){ cout << "the point is(" << _x << ", " << _y << ")" << endl;}     void print(){ _x = 1; _y = 1; }     void operator+=(const point &other){         _x = other._x + 100;         _y = other._y + 100;     }  private:     int _x;     int _y; };  int main() {     point p1;     point p2;     p1 +=p2;     p2.show();     getchar(); } 

its work. when change to:

point p1; point p1; point p2; point p3; p1 +=p2 +=p3; 

its doesnt , need return (*this). why it? if can explain me greatfull.. :)

the reason need return reference here p2 +=p3 not evaluate p2 after p3 has been added it. evaluate return value of operater+= of point void. cannot add void p1 error.

another way of looking @ p2 +=p3 p2.operator+=(p3). here can see not p2 instead p2.operator+=(p3) returns.

this why return reference. allows chain results trying do. if not return reference have break chain.