如何结识部分专门的模板类的朋友？

Question

I require a template specialised class to make friend with another template specialised class.

Example:

template <class A, class B>
class X { };

template <class A, class B>
class Y { };

Now I want to have only some specific specialisations of these template classes to be friends:

I tried

template <class A>
class X <A, bool> { };

template <class A>
class Y <A, bool>
{
    // X<A, bool> shall be friend!
    template<class _C> using X_partial = X<_C,bool>;
    template<class _D> friend class X_partial;

};

and this,

template <class A>
class X <A, bool> { };

template <class A>
class Y <A, bool>
{
    // X<A, bool> shall be friend!
    template<class C> friend class X<class C,bool> 

};

But both gives the error .

The syntax can support C++(03), C++11 or C++14.

Answer 1

The friend declaration cannot declare a partial specialization. See here .

Friend declarations shall not declare partial specializations. [ Example:

 template<class T> class A { }; class X { template<class T> friend class A<T*>; // error }; 

— end example ]

However, if the "initial" types for A are the same, then to provide friendship to the correct specialization, it is as follows;

template <class A, class B>
class Y { };

template <class A, class B>
class X {
public:
    // fails to compile when instantiated
    //X() { Y<A, bool> y; y.i = 42; }
};

template <class A>
class Y <A, bool>
{
    // partial specialisation provided friendship
    friend class X<A, bool>;
    int i = 0;
};

template <class A>
class X <A, bool> {
public:
    X() { Y<A, bool> y; y.i = 42; }
};

int main()
{
    X<int, bool> x1;
    // fails to compile.
    X<int, int> x2;
}

Sample online .

Answer 2

Where is a problem in that? What have you tried before asking this question?

// Templated class X
template <class A, bool>
class X {

};

// Templated class Y
template <class A, bool>
class Y {
    // Any specialized A is a friend of any specialized B
    template <class B, bool> friend class X;
};

Edit: Update with classes you provided in comment on your question.