template nested private class as a friend

Question

I have the following code that doesn't compile on GCC 4.9 as well as GCC 5.1. And I can't seem to figure out why. Sorry if this is a noob question, I am kind of new to C++ templates.

template<class T>class A
{
    template<class X>friend class B;
    struct C {};
};

template<class X>class B 
{
    template<class T>friend struct A<T>::C;
};

int main()
{
    A<int> a;
    B<float> b;
}

When compiled I get the following errors

root@localhost# g++-49 templatefriend.cpp
templatefriend.cpp: In instantiation of âclass B<float>â:
templatefriend.cpp:38:9:   required from here
templatefriend.cpp:27:9: error: âstruct A<T>::Câ is private
  struct C {};
     ^
templatefriend.cpp:31:1: error: within this context
 {
 ^

where as this compiles fine if I remove the templates

class A
{
    friend class B;
    class C{};
};

class B
{
    friend class A::C;
};

int main()
{
    A a;
    B b;
}

Any help is appreciated, or if such a question has already been asked, please share the link.

Answer 1

The warning you get from clang in this case is a bit more helpful:

warning: dependent nested name specifier 'A<X>::' for friend class declaration is not supported

In other words, A::C is a dependent type, so it doesn't work (though I don't off-hand know where in the standard this is described.

What I suspect is that in reality you only want the relationship to be between A<T> and B<T> where T is the same (eg A<int> and B<int> but not A<char> and B<float> ). If that is the case you can accomplish this by using the same template parameter in the friend declaration

template <typename T> class B;

template<class T>
class A {
    friend class B<T>;  // B of the same templated type is my friend
    struct C {};
};

template<class T>
class B {
    friend struct A<T>::C; // A::C of the same templated type is my friend
};

The other alternative is to have template <typename> class A; inside of B , which would then also make A::C a friend.