C++ Ostream Operator Overload - odd undefined reference when building - c++

Question

I'm doing a tutorial on C++ to really dig into the nitty-gritty of it, because frankly it's an awesome and powerful language. I'm still fairly new at it though, and I'm having issues with my operator overloading for the ostream operator.
Since this is a tutorial I'm trying to keep my code as organized and modular as possible to avoid confusion. There are 3 layers to my code:
main.cpp - runs the Exercises.
Exercises.cpp/Exercises.h - contains static functions for the exercises.
ComplexNumber.cpp/ComplexNumber.h - represents a complex number object in line with the tutorial.
Main is fairly simple: set of calls to Exercises::ExerciseName().
Exercises.cpp:
#include "Exercises.h"
void Exercises::ComplexNumberClass() {
ComplexNumber c; /** Runs the blank constructor */
ComplexNumber c1(2, 3); /** Runs the args-filled constructor */
ComplexNumber c2 = c1; /** Runs the copy constructor */
c = c2;
cout << c1 << endl;
}
The header file only has the header for that function as a public static void function, i.e. callable and executable. The ComplexNumber files are where I think the problem is (please correct me if I'm wrong):
ComplexNumber.cpp (without constructors and accessors):
#include "ComplexNumber.h"
using namespace ComplexNumbers;
using namespace std;
ostream& operator<< (ostream& out, ComplexNumber& num){
out << "(" << num.GetReal() << ", " << num.GetImaginary() << ")";
return out;
}
const ComplexNumber &ComplexNumber::operator=(const ComplexNumber &num) {
cout << "Using assignment operator override" << endl;
real = num.real;
imaginary = num.imaginary;
return *this;
}
ComplexNumber.h:
#ifndef OPERATOROVERLOADING_COMPLEXNUMBER_H
#define OPERATOROVERLOADING_COMPLEXNUMBER_H
#include <iostream>
using namespace std;
namespace ComplexNumbers {
class ComplexNumber {
private:
double real;
double imaginary;
public:
ComplexNumber();
ComplexNumber(double real, double imaginary);
ComplexNumber(const ComplexNumber &num);
const ComplexNumber &operator=(const ComplexNumber &num);
double GetReal() const {
return real;
}
double GetImaginary() const {
return imaginary;
}
};
ostream &operator<<(ostream& out, ComplexNumber& num);
}
#endif //OPERATOROVERLOADING_COMPLEXNUMBER_H
When I build this, it's telling me that I have an "undefined reference to 'cave_of_programming::operator<<(std::ostream&, cave_of_programming::ComplexNumber&)'. The method headers match, and I'm building with Jetbrains CLion.
My research suggests that the problem is in the linker, but I've looked at my CMakeLists.txt and found that it is linking up all of the appropriate files.
SOURCE_FILES main.cpp Exercises.cpp Exercises.h ComplexNumbers/ComplexNumber.cpp ComplexNumbers/ComplexNumber.h
Does anybody have an idea as to why this isn't working? What am I missing?

Answer 1

The problem is that you defined ostream &operator<<(ostream& out, ComplexNumber& num) inside the class. This doesn't make much sense to the compiler because it knows to override the operator<< when it gets an ostream object and a class of some type,
but the way you implemented it, it gets the ostream, a ComplexNumber, and another ComplexNumber as the 'this' reference.
In order to correctly overload a binary operator you have two options:
Define the operator inside the class, and give it only the ostream as parameter
class ComplexNumber
{
...
ostream &operator<<(ostream& out) const;
};
Or define the operator outside the class and give it both the parameters:
class ComplexNumber
{
...
};
ostream &operator<<(ostream& out, const ComplexNumber& num);
Note that in the second implementation, you need to add the function as a friend of the class if you want to access any private members.
Also note, that it's a good idea to use const here like I put in the examples, otherwise you won't be able to use the operator on const ComplexNumbers.