C++ -- 使用嵌套类中的类元素？

好的，我有一个类LinkedList，它有一个嵌套类LinkedListIterator。在LinkedListIterator的方法中，我引用了LinkedList的私有字段。我认为这是合法的。但我得到了错误：

from this location

每次我提到他们。

我在封闭类中的字段上得到了相应的错误消息：

invalid use of non-static data member 'LinkedList<int>::tail'

知道为什么吗？相关代码如下：

template<class T>
class LinkedList {
    private:
        //Data Fields-----------------//
        /*
         * The head of the list, sentinel node, empty.
         */
        Node<T>* head;
        /*
         * The tail end of the list, sentinel node, empty.
         */
        Node<T>* tail;
        /*
         * Number of elements in the LinkedList.
         */
        int size;
    class LinkedListIterator: public Iterator<T> {
            bool add(T element) {
                //If the iterator is not pointing at the tail node.
                if(current != tail) {
                    Node<T>* newNode = new Node<T>(element);
                    current->join(newNode->join(current->split()));
                    //Move current to the newly inserted node so that
                        //on the next call to next() the node after the
                        //newly inserted one becomes the current target of
                        //the iterator.
                    current = current->next;
                    size++;
                    return true;
                }
                return false;
            }