c++ multiple definitions of operator<<

Question

I am attempting to override the << operator for a class. The purpose is basically to implement a toString() like behavior for my class, so that sending it to cout will produce useful output. Using a dummy example, I have the code below. When I attempt to compile, I get the foollowing error:

$ g++ main.cpp Rectangle.cpp /tmp/ccWs2n6V.o: In function `operator<<(std::basic_ostream<char, std::char_traits<char> >&, CRectangle const&)': Rectangle.cpp:(.text+0x0): multiple definition of `operator<<(std::basic_ostream<char, std::char_traits<char> >&, CRectangle const&)' /tmp/ccLU2LLE.o:main.cpp:(.text+0x0): first defined here 

I can't figure out why this is happening. my code is below:

Rectangle.h:

#include <iostream> using namespace std;  class CRectangle {     private:         int x, y;         friend ostream& operator<<(ostream& out, const CRectangle& r);     public:         void set_values (int,int);         int area (); };  ostream& operator<<(ostream& out, const CRectangle& r){     return out << "Rectangle: " << r.x << ", " << r.y; } 

Rectangle.cpp:

#include "Rectangle.h"  using namespace std;  int CRectangle::area (){     return x*y; }  void CRectangle::set_values (int a, int b) {     x = a;     y = b; } 

main.cpp:

#include <iostream> #include "Rectangle.h"  using namespace std;  int main () {     CRectangle rect;     rect.set_values (3,4);     cout << "area: " << rect.area();     return 0; } 

Answer 1

You're breaking the one definition rule. A quick-fix is:

inline ostream& operator<<(ostream& out, const CRectangle& r){     return out << "Rectangle: " << r.x << ", " << r.y; } 

Others are:

• declaring the operator in the header file and moving the implementation to Rectangle.cpp file.
• define the operator inside the class definition.

.

class CRectangle {     private:         int x, y;     public:         void set_values (int,int);         int area ();         friend ostream& operator<<(ostream& out, const CRectangle& r){           return out << "Rectangle: " << r.x << ", " << r.y;         } }; 

Bonus:

• use include guards
• remove the using namespace std; from the header.