Overloading operator<< c++; undefined reference to `std::basic_ostream - c++

Question

I am trying to overload operator<< in my code. If I comment out the lines where I try to use the << operator with my custom class, it compiles fine. The error almost looks like it doesn't like the c++ libraries (?).
All my research on this problem has stated that it is a linking problem. Most suggest using g++ instead of gcc. I am using g++ as my compiler, and I get this error still.
Code:
#include <iostream>
using namespace std;
//prototype the class and the functions
template<class T> class strange;
template<class T> ostream& operator<< (ostream& osObject, strange<T>& sObject);
//begin class
template <class T>
class strange
{
public:
// .... function prototypes go here.
strange(T x,T y);
friend ostream& operator<< <> (ostream& osObject, strange<T>& sObject);
private:
T a;
T b;
};
// .... your function definitions go here
template <class T>
strange<T>::strange(T first, T second){
a = first;
b = second;
}
template <class T>
ostream& operator<< (ostream& osObject, const strange<T>& sObject){
osObject << sObject.a << ", " << sObject.b;
return osObject;
}
int main()
{
strange<int> x1(4,6) , x2(12,2) ;
//strange<char> y1('m','n') , y2('m','n') ;
cout << "x1 = " << x1 << endl;
return 0;
}
Error:
test.cpp:(.text+0x7a): undefined reference to `std::basic_ostream<char, std::char_traits<char> >& operator<< <int>(std::basic_ostream<char, std::char_traits<char> >&, strange<int>&)'
collect2: ld returned 1 exit status
Any idea what is causing this?

Answer 1

I made two changes, one to the friend definition, and one to the prototype. This should compile:
#include <iostream>
using namespace std;
//prototype the class and the functions
template<class T> class strange;
template<class T> ostream& operator<< (ostream& osObject, const strange<T>& sObject);
//begin class
template <class T>
class strange
{
public:
// .... function prototypes go here.
strange(T x,T y);
friend ostream& operator<< <> (ostream& osObject, const strange<T>& sObject);
private:
T a;
T b;
};
// .... your function definitions go here
template <class T>
strange<T>::strange(T first, T second){
a = first;
b = second;
}
template <class T>
ostream& operator<< (ostream& osObject, const strange<T>& sObject){
osObject << sObject.a << ", " << sObject.b;
return osObject;
}
int main()
{
strange<int> x1(4,6) , x2(12,2) ;
//strange<char> y1('m','n') , y2('m','n') ;
cout << "x1 = " << x1 << endl;
return 0;
}
And this compiles in clang, g++ and at ideone
To explain the issue, the compiler is looking at link time for a definition for:
std::ostream & operator<< <int>(std::ostream &, strange<int>&);
When you only have a definition for:
std::ostream & operator<< <int>(std::ostream &, strange<int> const &);
This is because of the miscommunication between your prototypes (both the explicit one and the friend) and your definition.