tinystl实现（第五步：itearator.h）

经过长时间的学习终于可以开始tinystl的仿（chao）写工作了，本文参考了这位大佬的github，坦白讲我只是补充了注释，因为tinystl的代码真的非常经典而我又没什么这种大型项目的经验，所以只能这样做，不过相信能够有助于大家的学习
#强烈建议按顺序阅读本专栏
迭代器是stl中各种容器实现的重中之重，我们发现平时常用的iterator居然又如此多种不同的必要形态，确实非常有趣
关于不同itearator的区别：查看这个文档

#pragma once
#ifndef _ITEARATOR_H_
#define _ITEARATOR_H_
#include<stddef.h>
namespace mySTL {

	struct input_iterator_tag {};//只读且读后往后移动
	struct output_iterator_tag {};//只写且写后往后移动
	struct forward_iterator_tag :public input_iterator_tag {};//可以从前往后遍历的指针
	struct bidirectional_iterator_tag : public forward_iterator_tag {};//从两个方向都可以遍历的指针
	struct random_access_iterator_tag : public bidirectional_iterator_tag {};//提供和指针相同的功能,换而言之它可以跳跃而不是逐个遍历

	template <class T, class Distance> struct input_iterator
	{
		typedef input_iterator_tag iterator_category;//迭代器本身
		typedef T value_type;//定义类型
		typedef Distance difference_type;//大小
		typedef T* pointer;//定义指针
		typedef T& reference;//定义引用
	};
	struct output_iterator
	{
		typedef output_iterator_tag iterator_category;
		typedef void value_type;
		typedef void difference_type;
		typedef void pointer;
		typedef void reference;
	};
	template <class T, class Distance> struct forward_iterator
	{
		typedef forward_iterator_tag iterator_category;
		typedef T value_type;
		typedef Distance difference_type;
		typedef T* pointer;
		typedef T& reference;
	};
	template <class T, class Distance> struct bidirectional_iterator
	{
		typedef bidirectional_iterator_tag iterator_category;
		typedef T value_type;
		typedef Distance difference_type;
		typedef T* pointer;
		typedef T& reference;
	};
	template <class T, class Distance> struct random_access_iterator
	{
		typedef random_access_iterator_tag iterator_category;
		typedef T value_type;
		typedef Distance difference_type;
		typedef T* pointer;
		typedef T& reference;
	};
	template<class Category, class T, class Distance = ptrdiff_t,
		class Pointer = T * , class Reference = T & >
		struct iterator
	{
		typedef Category iterator_category;
		typedef T value_type;
		typedef Distance difference_type;
		typedef T* pointer;
		typedef T& reference;
	};
	template<class Iterator>
	struct iterator_traits
	{
		typedef typename Iterator::iterator_category iterator_category;
		typedef typename Iterator::value_type value_type;
		typedef typename Iterator::difference_type difference_type;
		typedef typename Iterator::pointer pointer;
		typedef typename Iterator::reference reference;
	};
	template<class T>
	struct iterator_traits<T*>
	{
		typedef random_access_iterator_tag iterator_category;
		typedef T value_type;
		typedef ptrdiff_t difference_type;
		typedef T* pointer;
		typedef T& reference;
	};
	template<class T>
	struct iterator_traits<const T*>
	{
		typedef random_access_iterator_tag iterator_category;
		typedef T value_type;
		typedef ptrdiff_t difference_type;
		typedef T* pointer;
		typedef T& reference;
	};
	//以下三条函数均使用强制转型提取iterator中的元素
	template<class Iterator>
	inline typename  iterator_traits<Iterator>::iterator_category
		iterator_category(const Iterator& It) {
		typedef typename iterator_traits<Iterator>::iterator_category category;
			return category();
	}
	template<class Iterator>
	inline typename  iterator_traits<Iterator>::value_type*
		value_type(const Iterator& It) {
		return static_cast<typename iterator_traits<Iterator>::value_type*>(0);
	}
	template<class Iterator>
	inline typename  iterator_traits<Iterator>::diffence_type*
		diffence_type(const Iterator& It) {
		return static_cast<typename iterator_traits<Iterator>::diffence_type*>(0);
	}
}
#endif // !_ITEARATOR_H_