Correct way to implement operator!= for templates outside of class as a free function

Question

I have a following template with inner classes in a map.hpp file:

template<typename Key_T, typename Mapped_T>
class Map {
    // public members of Map ...

    class Iterator {
        //public members of Iterator ...

        friend bool operator!=(const Iterator &i, const Iterator &j) {
            return (i.link != j.link);  
        }

        // private members of iterator ...
        Node * link;
    };
};
#include "map.hxx"   //implementation file for member methods is separate

In the main.cpp I call the following and so far everything works fine:

Map<int, int> x;
// bunch of insertions ...
for (auto it = x.begin; it != x.end(); ++it) {
    // Do something with it ...   
}

However, I want to move the friend function out of the map.hpp file and into the map.hxx file which contains other implementation.

Q: Is it possible to move the free function to a .hxx file and how?

I tired declaring the function as a friend in the Iterator class and did the following in the implementation file:

template<typename Key_T, typename Mapped_T>
bool operator!=(const typename Map<Key_T, Mapped_T>::Iterator & i,
                const typename Map<Key_T, Mapped_T>::Iterator & j) {
    return (i.link != j.link);
}

However it failed with:

$clang++ -std=c++11 -stdlib=libc++ -Wall -Wextra -g main.cpp 

Undefined symbols for architecture x86_64:
  "shiraz::operator!=(shiraz::Map<int, int>::Iterator const&, shiraz::Map<int, int>::Iterator const&)", referenced from:
      _main in main-3oCRAm.o
ld: symbol(s) not found for architecture x86_64
clang: error: linker command failed with exit code 1 (use -v to see invocation)

Thanks!

Answer 1

A friend declaration without a corresponding match will inject a new function into the enclosing namespace. This function can only be found via ADL

The easiest way to do what you want is to leave a stub implementation in the iterator class, and have it forward to a "real" function outside. That keeps the nice ADL-only lookup of !=, which solves some thorny overload problems easily. The helper function can simply be a template<class Iterator> and be friended more normally and have a narrow implementation, and not be found via ADL but rather with a fully qualified lookup. Or it could be a static member function of the enclosing map.

template<typename Key_T, typename Mapped_T>
class Map {
  class Iterator;
  static bool it_equal( Iterator const&, Iterator const& );
  class Iterator {
    friend class Map;
    friend bool operator!=(const Iterator &i, const Iterator &j) {
      return !Map::it_equal(i,j);
    }
    friend bool operator==(const Iterator &i, const Iterator &j) {
      return Map::it_equal(i,j);
    }
    Node * link;
  };
};

now your .hxx simply implements bool Map<blah>::it_equal(blah) and you are done.