c++ multiple definitions of operator<< - c++

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.

Answer 2

You are putting the definition of a function in a .h file, which means that it will appear in every translation unit, violating the One Definition Rule (=> you defined operator<< in every object module, so the linker doesn't know which is "the right one").
You can either:
write just the declaration of your operator (i.e. its prototype) in the .h file and move its definition to rectangle.cpp
make operator<< inline - inline functions are allowed to be defined more than once, as long as all the definitions are identical.
(Also, you should use header guards in your includes.)