c++ multiple definitions of operator<<

问题:

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;
}

回答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.

回答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.)