operator overloading - inline non-member functions

Question

OK, so I can get my code to work, but there's something that's bugging me. It has to do with operator overloading and making non-member functions inline. Here's a very simple program that implements a complex number object:

Contained in Complex.h

using namespace std;

class Complex {
 private:
  double real;
  double imaginary;

 public:

  Complex(void);
  Complex(double r, double i);
  double getReal();
  double getImaginary();
  string toString();
};

inline Complex operator+(Complex lhs, Complex rhs);

...and in Complex.cc

#include <sstream>
#include <string>
#include "Complex.h"

using namespace std;

Complex::Complex(void)
{
...not important...
}

Complex::Complex(double r, double i)
{
  real = r;
  imaginary = i;
}

double Complex::getReal()
{
  return real;
}

double Complex::getImaginary()
{
  return imaginary;
}

string Complex::toString()
{
...what you would expect, not important here...
}


inline Complex operator+(Complex lhs, Complex rhs)
{
  double result_real = lhs.getReal() + rhs.getReal();
  double result_imaginary = lhs.getImaginary() + rhs.getImaginary();

  Complex result(result_real, result_imaginary);

  return(result);
}

and finally in plus_overload_test.cc

using namespace std;

#include <iostream>
#include "Complex.h"

int main(void)
{
  Complex c1(1.0,3.0);
  Complex c2(2.5,-5.2);

  Complex c3 = c1 + c2;

  cout << "c3 is " << c3.toString() << endl;

  return(0);
}

Compiling with g++ using a makefile that does the linking this produces the error:

plus_overload_test.cc:(.text+0x5a): undefined reference to `operator+(Complex, Complex)'

If I just remove the "inline" from before the operator+ in Complex.h and Complex.cc then everything compiles and works as it should. Why does the inline modifier cause this error? Everyone, for example:

Operator overloading

and

http://en.cppreference.com/w/cpp/language/operators

seems to recommend that for overloading binary operators the functions should be non-member and inline. So why am I encountering an error when I make them inline?

And, yes, I realize that the inline modifier may be a red herring since modern compilers should take care of this. But I remain curious.

Cheers!

Answer 1

An inline function must be defined in every file where it's used.

In case you want the precise wording from the standard (§7.1.2/4):

An inline function shall be defined in every translation unit in which it is odr-used and shall have exactly the same definition in every case.

With it marked inline, but defined in only one translation unit, you weren't meeting your side of your contract with the compiler (so to speak).