Question

The following code works, but i would like to move ostream& operator<< outside of the class declearation as i have with hash::operator[].

#include<iostream>
#include<map>

using namespace std;

template <class T>
class hash {
  private:
    map<string, T> _map;
  public:
    T& operator[] (string x);
    friend ostream& operator<<(ostream& out, const hash<T> &rhs) { return out << "test"; }
};

template <class T>
T & hash<T>::operator[](string x) {
  return _map[x];
}

int main () {
  hash<int> myobject;
  myobject["a"] = 1;
  cout << myobject["a"] << endl;
  cout << myobject << endl;
  return 0;
}

I have tried:

template <class T>
ostream& operator<<(...) {
  return out << "test";
}

and

ostream& operator<<(...) {
  return out << "test";
}

as well as a few other combinations to no avail.

Answer 1

You have to be careful when you use friend and template

#include<iostream>
#include<map>

template <class T>
class MyHash{
public:
    // ... use your T template here
    template <class U>
    friend ostream& operator<<(ostream& out, const MyHash<U> &rhs);
};

// ...
template <class U>
ostream& operator<<(ostream& out, const MyHash<U> &rhs){ return out << "test"; }

you need to use a different template U because operator<< is a friend method (external), if we use T instead of U, we will have : ambiguous definition in template .

Change hash by MyHash to avoid ambiguities.

Edit :

The error that you get here is :

error C2676: ‘<‘ binaire : ‘std::string’ ne définit pas cet opérateur
ou une conversion vers un type acceptable pour l’opérateur prédéfini

because you forgot to include <string> in “hash.h”. That header defines the < operator.

And also try moving the definition of operator[] and operator<< directly into “hash.h”
Both the declaration and definition of templates must be included. This is because in some compilers template functions cannot be compiled and linked independently, since they are generated on request for the specific types they are instantiated with.

Change the #include “hash.cpp” to #include “hash.h”