如何将模板实例化声明为班级的朋友？

Question

My node class needs the corresponding linked class to be a friend. I wrote it as

template <typename T>
class node{
    T value;
    node<T> *next;
    friend class linked<T>;
};

template <typename T>
class linked{
    linked();
    ~linked();
    node<T> *head;
};

I get a compilation error complaining that linked is not a class template. How can I declare linked<T> to be a friend of node<T> ?

Answer 1

If you want to make the instantiation of linked with the same template parameter T to be the friend, you need to forward declare class template linked at first.

// forward declaration
template <typename T>
class linked;

template <typename T>
class node {
    ...
    friend class linked<T>; 
    // or since C++11 you can
    friend linked<T>; 
};

template <typename T>
class linked {
    ...
};

Answer 2

You have to declare the class linked as follows ahead of using it in Node class. But your code looks more messy than just this issue. And I am not sure what you are actually attempting.

template <typename T>
class linked;

template <typename T>
class node{
    private:
    T value;
    node<T> *next;
    friend class linked<T>;
};

Answer 3

The error is that where the statement friend class linked<T>; appears, the linked class has not been declared. Add the following before class node .

template <typename T>
class linked;