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).