Перегрузка оператора --

#include<iostream>
using namespace std;
template<typename T>
class List {
private:
    struct Node {
    T data;
    Node *next, *prev;
    Node(const T &x):prev(NULL), next(NULL), data(x){}
};
    Node *first, *last;
    int count;
public:
    List(): first(NULL), last(NULL), count(0){}
    void push_back(const T d);
    void display();
    void display1();
    int size()const{return count;}
    class iterator{
    private:
        Node *pointer;
    public:
        iterator(Node *p = NULL):pointer(p){}
        T& operator*() {
            return pointer->data;
        }
        bool operator==(iterator other) {
            return pointer == other.pointer;
        }
        bool operator!=(iterator other) {
            return pointer != other.pointer;
        }
        iterator operator++() {
            if(pointer) return pointer = pointer->next;
        }
        iterator operator--(){
            if(pointer) return ;
        }
    };
    iterator begin() {
        return iterator(first);
    }
    iterator end() {
        return iterator(NULL);
    }
    iterator begin1() {
        return iterator(NULL);
    }
    iterator end1() {
        return iterator(last);
    }
};
 
template<typename T>
void List<T>::push_back(const T d){
    if(!first){
        first = last = new Node(d);
    }
    else{
        last->next = new Node(d);
        last->next->prev = last;
        last = last->next;
    }
    count++;
}
 
template<typename T>
void List<T>::display1(){
    iterator p;
    for (iterator m = end(); m != begin(); --m)
        if (m != end())
        cout << *m << " ";
    cout << endl << "----------------display ended----------------" << endl;
};
template<typename T>
void List<T>::display(){
    iterator p;
    for (iterator m = begin(); m != end(); ++m)
        cout << *m << " ";
    cout << endl << "----------------display ended----------------" << endl;
};
int main(){
    int num,a;
    List<int> xxx;
    cout << "----------------Fanny program----------------"<< endl;
    cout << "Input quantity of elements:"<< endl;
    cin >> num ;
    for (int i = 0; i < num; i++){
        cout << "Input your number: "<< endl;
        cin >> a;
        xxx.push_back(a);
    }
    cout << "Output of array: " << endl;
//  xxx.push_back(1);
//  xxx.push_back(2);
//  xxx.push_back(3);
    xxx.display1();
    system("PAUSE");
    return 0;
}