Template friend error, error: template argument required for 'class LinkedList'

Question

Hi I am tryna friend a class linkedlist in iterator but idk how to do it with templates

class Iterator{
    private:
        node<T>* v;

    public:
        Iterator(node<T>* u){
            v = u;
        }

        T& operator*(){
            return v->element; 
        }
        friend class LinkedList;


};

template <class T>
class LinkedList{
    public:
        node<T>* head;
        node<T>* tail;
}

I keep getting an error

doubleLinkedList.cpp: In instantiation of 'class Iterator<int>':
doubleLinkedList.cpp:214:26:   required from here
doubleLinkedList.cpp:35:18: error: template argument required for 'class LinkedList'
         node<T>* v;
                  ^

I have tried alot of things like i putted a template on top of friend argument but nothing works

I presume your iterator is also a template. Does it work if you do `friend class LinkedList` ? — Joseph Larson, Oct 17 '21 at 18:42
You can only leave out template arguments within the definition of the template itself. — molbdnilo, Oct 17 '21 at 18:56

score 0 · Answer 1 · answered Oct 17 '21 at 21:01

If you want to make sure that LinkedList is only a friend of Iterator if they share the same T, then you can forward declare LinkedList and make the specialization for T a friend:

template <class T> class LinkedList;

template <class T>
class Iterator{
    friend class LinkedList<T>;
};

template <class T> class LinkedList{};

If you want every LinkedList to be a friend of iterator you could also write

template <class T>
class Iterator{
     template <class U> friend class LinkedList; // can't use T here
};

template <class T> class LinkedList{};

But LinkedList<double> being a friend of Iterator<int> is probably not what you want. Why not template <class T> friend class LinkedList whithout forward declaration? That's because nested templates must have different template parameters: one shadowing the other is not allowed.

score 0 · Answer 2 · answered Oct 18 '21 at 02:41

There are a few issues (see version below to see them applied):

If a template is used (like node) the T must be defined like template <class T>

If you reference something, it must have been declared first. Declared is just the type, like int x;, class y;, or <template class T> class z; The definition -- all the details can come later: int x=7 or the full class definition. This is required for friend as well.

And, this still gets me sometimes, all classes must end their definition with a ;

template <class T> class node; // this has to be declared or defined first

template <class T>class LinkedList; // this is needed to reference as friend later

template <class T> // this is needed because to pass template args, the class must be templated as well 
class Iterator {
    private:
        node<T>* v;

    public:
        Iterator(node<T>* u) {
            v = u;
        }

        T& operator*() {
            return v->element; 
        }
        friend class LinkedList<T>;
};

template <class T>
class LinkedList{
    public:
        node<T>* head;
        node<T>* tail;
}; // remember the closing semi colon

Template friend error, error: template argument required for 'class LinkedList'

2 Answers2