C++ STL deque 的实现

#include <iostream>
using namespace std;
template <class T>class Deque {
protected:
	struct node {
		T data;
		node* next;
	};
	node* head;
	node* tail;
	size_t len;
public:
	Deque() {
		head = NULL;
		tail = NULL;
		len = 0;
	}
	~Deque() {
		node* ptr = NULL;
		while (head != NULL) {
			ptr = head->next;
			delete head;
			head = ptr;
		}
	}
	void push_back(T var) {
		node* ptr = new node;
		ptr->data = var;
		ptr->next = NULL;
		if (tail != NULL) {
			tail->next = ptr;
		}
		else {
			head = ptr;
		}
		tail = ptr;
		len++;
	}
	void push_front(T var) {
		node* ptr = new node;
		ptr->data = var;
		ptr->next = head->next;
		head = ptr;
		if (tail == NULL) {
			tail = ptr;
		}
		len++;
	}
	void pop_front() {
		if (len <= 0) {
			abort();
		}
		node* ptr = tail->next;
		delete tail;
		tail = ptr;
		if (tail == NULL) {
			head = NULL;
		}
		len--;
	}
	void pop_back() {
		if (len <= 0) {
			abort();
		}
		node* ptr = head->next;
		delete head;
		head = ptr;
		if (head == NULL) {
			tail = NULL;
		}
		len--;
	}
	T front() {
		return head->data;
	}
	T back() {
		return tail->data;
	}
	bool empty() {
		return head == NULL && tail == NULL;
	}
	size_t size() {
		return len;
	}
	void clear() {
		while (len > 0) {
			pop_back();
			pop_front();
		}
	}
};