undefined reference to operator<<

Question

I have a regular class (not a template, that is) with a private friend operator<<

it\'s declaration is:

std::ostream& operator<<(std:         


        

Answer 1

David gave a very good answer to the question. Here I am just providing an example for easy understanding.

in my header file:

namespace BamTools {
class SomeBamClass {
.....
public:
    friend std::ostream& operator<<(std::ostream& ous, const SomeBamClass& ba);
    .....
}
}

In the cpp file:

using namespace BamTools; // this work for Class member function but not friends
using namespace std;

........
std::ostream& operator<<(std::ostream &ous, const SomeBamClass &ba) {
  // actual implementation
  return ous;
}

The above will give you undefined reference to operator<<(.., const SomeBamClass&) when you try to link to this library from your own applications because it is declaration and implementation of an operator outside the namespace.

undefined reference to `BamTools::operator<<(std::ostream&, BamTools::SomeBamClass const&)'

The only thing works for me is to add namespace inclosure to the friend function in the implementation file:

namespace BamTools {
std::ostream& operator<<(std::ostream &ous, const SomeBamClass &ba) {
...
   return ous;
}
}

using ... BamTools::operator<<(..., const BamTools::SomeBamClass&) will result in a compiler error for my g++ 5.4.0.