C++ STL stack 的实现

#include <iostream>
using namespace std;
#define ulli size_t
template<class T>class Stack {
private:
	struct Node {
		T data;
		Node* next;
	};
	Node* head;
	Node* p;
	ulli length;

public:
	Stack() {
		head = NULL;
		length = 0;
		p = NULL;
	}
	void push(T n) {
		Node* q = new Node;
		q->data = n;
		if (head == NULL) {
			q->next = head;
			head = q;
			p = q;
		}
		else {
			q->next = p;
			p = q;
		}
		length++;
	}
	void pop() {
		if (length <= 0) {
			abort();
		}
		Node* q;
		T data;
		q = p;
		data = p->data;
		p = p->next;
		delete(q);
		length--;
	}
	ulli size() {
		return length;
	}
	T top() {
		return p->data;
	}
	bool empty() {
		if (length == 0) {
			return true;
		}
		else {
			return false;
		}
	}
	void clear() {
		while (length > 0) {
			pop();
		}
	}
};