partial template specialization for friend classes?

Question

I have a simple class X, and set of templatized classes Y. I\'d like all classes Y where the first templatization parameter happens to be X to be a friend of X i

Answer 1

The friend declaration page on cppreference.com specifies:

Friend declarations cannot refer to partial specializations, but can refer to full specializations

So as chtz said you can have a non-partial specialization friend.

Edit:

See also another answer on stackoverflow: https://stackoverflow.com/a/11046918/5776353

Answer 2

For the non-partial part of your question, the syntax is:

class X {
    template<class T, class U> friend class Y;
};

I guess, in most cases that should be sufficient.

---

With C++11 you can actually friend a templated alias:

template<typename T, typename U>
class Y { };

class X {
    public:
        X(int value) : i(value) {}
        const int& getI()    const { return i; }
    private:
        int    i;
        template<class U> using YX = Y<X,U>;
        template<class U> friend class YX;
};

However, that does not seem to work (I'm not sure if the friend declaration above has any effect at all).