C++ templates and friends, linker error

Question

Why does the following code not compile (linker error, unresolved external symbol, class std::basic_ostream > & __cdecl

Answer 1

This line:

friend std::ostream& operator<<(std::ostream& Stream, const Class& Op);

Should be:

template <class T> friend std::ostream& operator<<(std::ostream& Stream, const Class<T>& Op);