Застрял на итераторной реализации Trie

Мне нужно реализовать самодельный Trie, и я застрял на части Iterator. Кажется, я не могу понять метод приращения для дерева.

Я надеюсь, что кто-нибудь поможет мне разобраться.

Вот код итератора:

template <typename T> class Trie<T>::IteratorPrefixe{
friend class Trie<T>;
public:
    IteratorPrefixe() : tree(NULL), currentNode(NULL), currentKey("") {};
    pair<string, T*> operator*() {return make_pair(currentKey, currentNode -> element);} ;
    IteratorPrefixe operator++()throw(runtime_error);
    void operator=(IteratorPrefixe iter) {tree = iter.tree; currentNode = iter.currentNode; currentKey = iter.currentKey;};
    bool operator==(IteratorPrefixe iter) {return tree == iter.tree && currentNode == iter.currentNode;};
    bool operator!=(IteratorPrefixe iter) {return tree != iter.tree || currentNode != iter.currentNode;};
private:
    Trie<T> * tree;
    Trie<T> * currentNode;
    string currentKey;
};

А вот и мой Трие:

template <typename T> class Trie {
friend class IteratorPrefixe;
public:
    // Create a Trie<T> from the alphabet of nbletters, where nbletters must be
    // between 1 and NBLETTERSMAX inclusively
    Trie(unsigned nbletters) throw(runtime_error);

    // Add a key element of which is given in the first argument and content second argument
    // The content must be defined (different from NULL pointer)
    // The key is to be composed of valid letters (the letters between A + inclusive and exclusive nbletters
    //      Eg  if nblettres is 3, a, b and c are the only characters permitted;
    //          If nblettres is 15, only the letters between a and o inclusive are allowed.
    // Returns true if the insertion was achieved, returns false otherwise.
    bool addElement(string, T*) throw(runtime_error);
    // Deletes a key element of which is given as an argument and returns the contents of the node removed
    // The key is to be composed of letters valid (see above)
    // Can also delete at the same time the reference of the ancestors, if these ancestors are no longer used.
    // Returns NULL if the item has no delete
    T* removeElement(string cle) throw(runtime_error);
    // Find a key element of which is given as an argument and returns the associated content
    // The key is to be composed of letters valid (see above)
    // Returns NULL if the key does not exist
    T* searchElement(string cle) throw();
    // Iterator class to browse the Trie <T> in preorder mode
    class IteratorPrefixe;
    // Returns an iterator pointing to the first element
    IteratorPrefixe pbegin() throw(runtime_error);
    // Returns an iterator pointing beyond the last item
    IteratorPrefixe pend() throw();

private:
    unsigned nbLetters;
    T* element;
    vector<Trie<T> *> childs;
    Trie<T> * parent;

    // This function removes a node and its ancestors if became unnecessary. It is essentially the same work
    // as deleteElement that is how to designate remove a node that is changing. Moreover, unlike
    // deleteElement, it does not return any information on the node removed.
    void remove(Trie<T> * node) throw();

    // This function is seeking a node based on a given key. It is essentially the same work
    // searchElement but that returns a reference to the node found (or null if the node does not exist)
    // The key is to be composed of letters valid (see above)
    Trie<T>* search(string key) throw(runtime_error);
};