Nested private class template as friend
I have the following code that does not compile on GCC 4.9 as well as GCC 5.1. And I can't figure out why. Sorry if this is a noob question, I'm 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;
}
I am getting the following errors when compiling
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 does this compile 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.
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The warning you get from clang in this case is a little helpful:
warning: dependent nested name specifier 'A<X>::' for friend class declaration is not supported
In other words, A :: C is type dependent, so it doesn't work (although I don't know where in the standard it is described.
I suspect that you actually want the ratio between A<T>
and B<T>
, where is the T
same (for example, A<int>
and B<int>
, but not A<char>
and B<float>
). If so, you can accomplish this using the same template parameter in your friend's 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
};
Another alternative is template <typename> class A;
inside B
, which will then also make a A::C
friend.
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