c++ - Error when overloading insertion (<<) and addition (+) -

i'm learning c++ , baffling me. have vector class plus , insertion operators overloaded:

#include <iostream>  class vector {     public:         vector(float _x, float _y, float _z) {             x = _x; y = _y; z = _z;         }          float x, y, z; };  vector operator+(const vector &v1, const vector &v2) {     return vector(v1.x + v2.x, v1.y + v2.y, v1.z + v2.z); }  std::ostream &operator<<(std::ostream &out, vector &v) {     out << "(" << v.x << ", " << v.y << ", " << v.z << ")";      return out; }   int main() {     vector i(1, 0, 0);     vector j(0, 1, 0);      std::cout << i;     /* std::cout << (i + j); */  } 

when try print vector fine:

vector i(1, 0, 0); std::cout << i;  // => "(1, 0, 0)" 

adding vectors works fine also:

vector i(1, 0, 0); vector j(0, 1, 0); vector x = + j; std::cout << x;  // => "(1, 1, 0)" 

but if try print sum of 2 vectors without intermediary variable huge compile error don't understand:

vector i(1, 0, 0); vector j(0, 1, 0); std::cout << (i + j);  // compile error  vector.cpp: in function ‘int main()’: vector.cpp:28:15: error: no match ‘operator<<’ (operand types ‘std::ostream {aka std::basic_ostream<char>}’ , ‘vector’)      std::cout << (i + j);                ^ vector.cpp:17:15: note: candidate: std::ostream& operator<<(std::ostream&, vector&) <near match>  std::ostream &operator<<(std::ostream &out, vector &v) {                ^ vector.cpp:17:15: note:   conversion of argument 2 ill-formed: vector.cpp:28:21: error: invalid initialization of non-const reference of type ‘vector&’ rvalue of type ‘vector’      std::cout << (i + j); 

what doing wrong? should work?

the result of addition operator isn’t can take non-const reference to. since you’re not modifying vector in <<, though, can , should make const:

std::ostream &operator<<(std::ostream &out, const vector &v) {     out << "(" << v.x << ", " << v.y << ", " << v.z << ")";      return out; }