Программа не хочет работать. Списки

Программа не работает. Это моя первая практика, поэтому, конечно же, есть ошибки. Нуждаюсь в помощи.

#include <iostream>
using namespace std;
struct Double_node{ 
    int val;
    Double_node* next;
    Double_node* prev;
};
class Double_list {
private:
    Double_node* tail= nullptr;
    int sz=1;
public:
    Double_list() { tail = new Double_node();
    tail->next = tail;
    tail->prev = tail;
    }
    Double_list(int x) {
        Double_node* t = new Double_node();
        t->next = t;
        t->prev = t;
            t->val = x;
    }
    ~Double_list() {}
        void push_front(int x) {
            Double_node*cur = tail -> next;
            tail->next = new Double_node(x,cur,tail);
    }
        void push_back(int x) {
            Double_node* cur = tail->prev;
            tail->prev = new Double_node(x, tail, cur);
            }
        void pop_back() {
            Double_node* cur = tail->prev->prev;
            delete tail->prev;
            tail->prev = cur;
            cur->next = tail;
        }
        void pop_front() {
            Double_node* cur = tail->next->next;
            delete tail->next;
            tail->next = cur;
            cur->prev = tail;
        }

        bool empty() { 
            if (tail->next = tail) return true;
            else return false;
        }
        int size() {
            Double_node* cur = tail->next;
            while (cur != tail)
            {
                sz++;
                cur = cur->next;
            }
            return sz;
        }
        void clear() {}
        int& front() { return tail->next->val; }
        int& back() { return tail->prev->val; }
        void print() {
            Double_node* cur = tail->next;
            while (cur != tail)
            {
                cout << cur->val << endl;
                cur = cur->next;
            }
        }
    };

int main()
{
    Double_list a;
    Double_list zz;
    cout << zz.size;

    return 0;
}