模板朋友类：前瞻声明或......？

Question

Suppose I have a template class that I am trying to declare as a friend class. Should I forward declare the class or give it its own template?

Example:

template <typename E>
class SLinkedList;
template <typename E>
class SNode {
private:
  E elem;
  SNode<E>* next;
  friend class SLinkedList<E>;
};

Or

template <typename E>
class SNode {
private:
  E elem;
  SNode<E>* next;
  template <typename T>
  friend class SLinkedList;
};

Answer 1

Your first approach is probably what you want. It will make SLinkedList<int> a friend of SNode<int> , and similar for all matching types.

Your second approach will make every SLinkedList a friend of every SNode . This is probably not what you want as SLinkedList<Widget> has no business touching the private parts of an SNode<int>

---

A different approach I could recommend would be to make SNode a nested class. This is pretty common for data structures consisting of nodes:

template <typename E>
class SLinkedList {
    struct SNode {
        E elem;
        SNode* next;
    };
};

With this scheme you may as well get rid of the friend declaration and have everything in SNode be public, because the whole class would be private to SLinkedList