#include ＜iterator＞

C++ 标准库中的 <iterator> 头文件提供了一组工具，用于遍历容器中的元素。迭代器是 C++ 标准模板库（STL）中的核心概念之一，它允许程序员以统一的方式访问容器中的元素，而不需要关心容器的具体实现细节。

迭代器是一个对象，它提供了一种方法来遍历容器中的元素。迭代器可以被视为指向容器中元素的指针，但它比指针更加灵活和强大。迭代器可以用于访问、修改容器中的元素，并且可以与 STL 算法一起使用。

// <iterator> -*- C++ -*-

// Copyright (C) 2001-2020 Free Software Foundation, Inc.
//
// This file is part of the GNU ISO C++ Library.  This library is free
// software; you can redistribute it and/or modify it under the
// terms of the GNU General Public License as published by the
// Free Software Foundation; either version 3, or (at your option)
// any later version.

// This library is distributed in the hope that it will be useful,
// but WITHOUT ANY WARRANTY; without even the implied warranty of
// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
// GNU General Public License for more details.

// Under Section 7 of GPL version 3, you are granted additional
// permissions described in the GCC Runtime Library Exception, version
// 3.1, as published by the Free Software Foundation.

// You should have received a copy of the GNU General Public License and
// a copy of the GCC Runtime Library Exception along with this program;
// see the files COPYING3 and COPYING.RUNTIME respectively.  If not, see
// <http://www.gnu.org/licenses/>.

/*
 *
 * Copyright (c) 1994
 * Hewlett-Packard Company
 *
 * Permission to use, copy, modify, distribute and sell this software
 * and its documentation for any purpose is hereby granted without fee,
 * provided that the above copyright notice appear in all copies and
 * that both that copyright notice and this permission notice appear
 * in supporting documentation.  Hewlett-Packard Company makes no
 * representations about the suitability of this software for any
 * purpose.  It is provided "as is" without express or implied warranty.
 *
 *
 * Copyright (c) 1996,1997
 * Silicon Graphics Computer Systems, Inc.
 *
 * Permission to use, copy, modify, distribute and sell this software
 * and its documentation for any purpose is hereby granted without fee,
 * provided that the above copyright notice appear in all copies and
 * that both that copyright notice and this permission notice appear
 * in supporting documentation.  Silicon Graphics makes no
 * representations about the suitability of this software for any
 * purpose.  It is provided "as is" without express or implied warranty.
 */

/** @file include/iterator
 *  This is a Standard C++ Library header.
 */

#ifndef _GLIBCXX_ITERATOR
#define _GLIBCXX_ITERATOR 1

#pragma GCC system_header

#include <bits/c++config.h>
#include <bits/stl_iterator_base_types.h>
#include <bits/stl_iterator_base_funcs.h>
#include <bits/stl_iterator.h>
#include <iosfwd>
#include <bits/stream_iterator.h>
#include <bits/streambuf_iterator.h>
#include <bits/range_access.h>

#if __cplusplus >= 201402L && ! defined _GLIBCXX_DEBUG // PR libstdc++/70303
# define __cpp_lib_null_iterators 201304L
#endif

#endif /* _GLIBCXX_ITERATOR */

迭代器主要分为以下几类：

1. 输入迭代器（Input Iterator）：只能进行单次读取操作，不能进行写入操作。
2. 输出迭代器（Output Iterator）：只能进行单次写入操作，不能进行读取操作。
3. 正向迭代器（Forward Iterator）：可以进行读取和写入操作，并且可以向前移动。
4. 双向迭代器（Bidirectional Iterator）：除了可以进行正向迭代器的所有操作外，还可以向后移动。
5. 随机访问迭代器（Random Access Iterator）：除了可以进行双向迭代器的所有操作外，还可以进行随机访问，例如通过下标访问元素。