In C++, why isn't it possible to friend a template class member function using the template type of another class?

Question

In other words, why does this compile fine :

template
class A{
  public:
    void f();
};

class B{
  friend void A::f();
};

tem         


        

Answer 1

An explicit instantiation of B before it is used in A::f() resolves this problem. I assume GCC tries an implicit instantiation of B in the definition of A::f(). But the definition of A::f() is not finished and GCC 'looses' the friend declaration. It looks like a compiler problem.

template
class A
{
public:
    void f();
};

template // B is now a templated class
class B
{
    friend void A::f(); // Friending is done using B templated type
};

template
class B; // <= explicit instantiation, that works

template<>
void A::f()
{
    B* var = new B();
}