Templates & Linkerfehler

[interpreter]

Hi, ich bekomme bei diesem Code:

#include <iostream>
using namespace std;

template <class T>
class Foo {
private:
	T var;
public:
	Foo(T v):var(v) { }
	~Foo() { }
	T getVar() const { return var; }
	void setVar(T var) { this->var = var; }
	friend ostream& operator<<(ostream& o, const Foo& rhs);
	operator T*() const;
};

template <class T>
ostream& operator<<(ostream &o, const Foo<T>& rhs) {
	o << rhs.var;
	return o;
}

template <class T>
Foo<T>::operator T*() const {
	return &var;
}

int main() {
	Foo<int> f(10);
	cout << f;

	int i2;
	cin >> i2;
	return 0;
}

folgenden Linkerfehler:
main.obj : error LNK2019: unresolved external symbol "class std::basic_ostream<char,struct std::char_traits<char> > & __cdecl operator<<(class std::basic_ostream<char,struct std::char_traits<char> > &,class Foo<int> const &)" (??6@YAAAV?$basic\_ostream@DU?$char_traits@D@std@@@std@@AAV01@ABV?$Foo@H@@@Z) referenced in function _main
Debug/cpplearning.exe : fatal error LNK1120: 1 unresolved externals

Ich kann mit diesem Linkerfehler allerdings nichts anfangen. Was habe ich in meiner Klasse falsch gemacht?