After improving code from suggestions to my previous question. I have implemented copy constructor, copy assignment, move constructor, move assignmnet. There is a suggestion for sentinal node but I don’t know how to write code for that and I have used exit(0) in the end because the program was not terminating. I want to improve this code.

#include <iostream> #include <utility>  template <typename T> class circular_doubly_linked_list {   struct Node   {     T data;     Node * next;     Node * prev;     Node(T value) : data(std::move(value)),                     next(nullptr),                     prev(nullptr)                     {}   };   Node *head, *tail;  public:   circular_doubly_linked_list() : head(nullptr), tail(nullptr) {}   circular_doubly_linked_list(const circular_doubly_linked_list &); //copy constructor   circular_doubly_linked_list& operator=(const circular_doubly_linked_list& cdll) //copy assignment   {     circular_doubly_linked_list  temp(cdll);     temp.swap(*this);     return *this;   }   circular_doubly_linked_list(circular_doubly_linked_list&&) noexcept; //move constructor   circular_doubly_linked_list& operator=(circular_doubly_linked_list&& cdll) noexcept //move assignment   {     cdll.swap(*this);     return *this;   }   ~circular_doubly_linked_list();    void append_node(T);   void delete_node(T);    friend void swap(circular_doubly_linked_list& lhs, circular_doubly_linked_list& rhs)   {     std::swap(lhs.head, rhs.head);   }    template <typename U>   friend std::ostream & operator<<(std::ostream & os, const circular_doubly_linked_list<U> & cdll)   {     cdll.print_list(os);     return os;   }  private:    struct Node *search(T value)   {     Node *node = head;     while(node->next != head)     {       if(node->data == value)       {         return node;       }       node = node->next;     }     if(node->data == value)     {       return node;     }     return nullptr;   }    void print_list(std::ostream& os = std::cout) const   {     Node *tmp = head;     while(tmp->next != head)     {       std::cout << tmp->data << ' ';       tmp = tmp->next;     }     std::cout << tmp->data << '\n';   } };  template <typename T> circular_doubly_linked_list<T>::circular_doubly_linked_list(const circular_doubly_linked_list & cdll) {   if(cdll.head == nullptr)   {     head = tail = nullptr;   }   else   {     head = new Node(cdll.head->data);     Node *curr = head;     Node *tmp = head;     Node *obj_curr = cdll.head;     while(obj_curr->next != cdll.head)     {       curr->next = new Node(obj_curr->next->data);       obj_curr = obj_curr->next;       curr = curr->next;       curr->prev = tmp;       tmp = tmp->next;     }     tail = curr;     curr->next = head;     head->prev = curr;   } }  template <typename T> circular_doubly_linked_list<T>::circular_doubly_linked_list(circular_doubly_linked_list&& cdll) noexcept {   head = tail = nullptr;   swap(*this, cdll); }  template <typename T> void circular_doubly_linked_list<T>::append_node(T value) {   Node *node = new Node(std::move(value));    if(head == nullptr)   {     node->next = node;     node->prev = node;     head = node;     tail = node;   }    tail = head->prev;   tail->next = node;   node->prev = tail;   node->next = head;   head->prev = node;   tail = node; }  template <typename T> void circular_doubly_linked_list<T>::delete_node(T value) {   Node *node = search(value);   if(node == nullptr)   {     std::cerr << "No such value in the list\n";     return;   }   else   {   Node *tmp = head;   Node *tail = head->prev;   if(tmp == node)   {     tail->next = tmp->next;     tmp->prev->next->prev = tail;     head = tail->prev;     delete tmp;     return;   }   else if(tail == node)   {     Node *curr = tail;     tmp = tail->prev;     tmp->next = curr->next;     head->prev = tmp;     tail = tmp;     delete curr;     return;   }   else   {     while(tmp->next != head)     {       if(tmp == node)       {         tmp->prev->next = tmp->next;         tmp->prev->next->prev = tmp->prev;         delete tmp;         return;       }       tmp = tmp->next;     }   } }  }  template <typename T> circular_doubly_linked_list<T>::~circular_doubly_linked_list() {   Node *tmp = nullptr;   while(head!= nullptr)   {     tmp = head;     head = head->next;     delete tmp;   } }  int main() {   circular_doubly_linked_list<int> cdll1;   cdll1.append_node(3);   cdll1.append_node(4);   cdll1.append_node(5);   cdll1.append_node(6);   cdll1.append_node(7);   cdll1.append_node(8);   std::cout << cdll1;   cdll1.delete_node(6);   std::cout << cdll1;   circular_doubly_linked_list<int> cdll2(cdll1); // using copy constructor   std::cout << "Linked List 2: " << cdll2;   circular_doubly_linked_list<int> cdll3 = cdll1; //using copy assignment   std::cout << "Linked List 3: " << cdll3;   circular_doubly_linked_list<int> cdll4 = std::move(cdll2); //using move constructor   std::cout << "Linked list 4: " << cdll4;   exit(0); }