Contradictory Error Messages - Operator Overloading <<

Question

Problem

Depending on how I write a function in my class, I get one of 2 contadictory error messages when trying to overload the << operator as a friend of my class. The error messages are as follows:

// With two arguments
(engine.cpp) error: 'std::ostream& game::GameEngine::operator<<( std::ostream&, const Engine& )' must take exactly one argument.

Otherwise, I get this if I try what the compiler says:

// With 1 argument
(engine.h) error: 'std::ostream& game::operator<<( std::ostream& )' must take exactly two arguments.
(engine.cpp) error: no 'std::ostream& game::GameEngine::operator<<( std::ostream& )' member function declared in class 'game::GameEngine'

I'm of the opinion that the one with two arguments is the correct one, but I do not understand why I am getting contradictory error messages. I'm using `-Wall, -Wextra, -pedantic-error, -std=c++11' and several other warning flags to compile the file.

Code

engine.h source:

#include <iostream>
namespace game {
    // Note that all variables and functions except the operator overload are static.
    class GameEngine {
    public:
      /* ... */
      friend std::ostream& operator<<( std::ostream& o, const GameEngine& engine );
    private:
      /* ... */
    }
    typedef game::GameEngine Engine;

And, engine.cpp:

#include <iostream>
#include "engine.h"
/* ... */
std::ostream& Engine::operator<<( std::ostream& o, const Engine& engine ) {
    /* ... */
}

Answer 1

When you declare the function inside the class as a friend, it is a member of the enclosing namespace and not of the class itself. So you need to define it as

std::ostream& game::operator<<( std::ostream& o, const Engine& engine ) { ... }