std::counted_iterator

 

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迭代器库

迭代器概念
indirectly_readable (C++20) indirectly_writable (C++20) weakly_incrementable (C++20) incrementable (C++20) input_or_output_iterator (C++20) sentinel_for (C++20) sized_sentinel_for (C++20)	input_iterator (C++20) output_iterator (C++20) forward_iterator (C++20) bidirectional_iterator (C++20) random_access_iterator (C++20) contiguous_iterator (C++20)
迭代器原语
iterator_traits input_iterator_tagoutput_iterator_tagforward_iterator_tagbidirectional_iterator_tagrandom_access_iterator_tagcontiguous_iterator_tag (C++20) iterator (C++17 中弃用)	incrementable_traits (C++20) indirectly_readable_traits (C++20) iter_value_titer_difference_titer_reference_titer_rvalue_reference_titer_common_reference_t (C++20)(C++20)(C++20)(C++20)(C++20)
算法概念与工具
间接可调用概念
indirectly_unary_invocableindirectly_regular_unary_invocable (C++20)(C++20) indirect_unary_predicate (C++20)	indirect_binary_predicate (C++20) indirect_equivalence_relation (C++20) indirect_strict_weak_order (C++20)
常用算法要求
indirectly_movable (C++20) indirectly_movable_storable (C++20) indirectly_copyable (C++20) indirectly_copyable_storable (C++20)	indirectly_swappable (C++20) indirectly_comparable (C++20) permutable (C++20) mergeable (C++20) sortable (C++20)
工具
indirect_result_t	projected
迭代器适配器
reverse_iterator move_iterator (C++11) move_sentinel (C++20) common_iterator (C++20) counted_iterator (C++20) back_insert_iterator front_insert_iterator insert_iterator	make_reverse_iterator (C++14) make_move_iterator (C++11) default_sentinel_tdefault_sentinel (C++20)(C++20) unreachable_sentinel_tunreachable_sentinel (C++20)(C++20) back_inserter front_inserter inserter
流迭代器
istream_iterator ostream_iterator	istreambuf_iterator ostreambuf_iterator
迭代器定制点
ranges::iter_move (C++20)
ranges::iter_swap (C++20)
迭代器操作
advance distance prev (C++11) next (C++11)	ranges::advance (C++20) ranges::distance (C++20) ranges::prev (C++20) ranges::next (C++20)
范围访问
begincbegin (C++11)(C++14) endcend (C++11)(C++14) sizessize (C++17)(C++20)	rbegincrbegin (C++14)(C++14) rendcrend (C++14)(C++14) empty (C++17) data (C++17)

 

std::counted_iterator

成员函数

counted_iterator::counted_iterator (C++20)

counted_iterator::operator= (C++20)

counted_iterator::base (C++20)

counted_iterator::count (C++20)

counted_iterator::operator*counted_iterator::operator-> (C++20)(C++20)

counted_iterator::operator[] (C++20)

counted_iterator::operator++counted_iterator::operator++(int)counted_iterator::operator+counted_iterator::operator+=counted_iterator::operator--counted_iterator::operator--(int)counted_iterator::operator-counted_iterator::operator-= (C++20)(C++20)(C++20)(C++20)(C++20)(C++20)(C++20)(C++20)

非成员函数

operator==operator<=> (C++20)(C++20)

operator==(default_sentinel_t) (C++20)

operator+ (C++20)

operator- (C++20)

operator-(default_sentinel_t) (C++20)

iter_move (C++20)

iter_swap (C++20)

辅助类

iterator_traits (C++20)

 

定义于头文件 <iterator>
template< std::input_or_output_iterator I > class counted_iterator;		(C++20 起)

std::counted_iterator 是表现准确同底层迭代器的迭代器适配器，除了它记录到其范围末尾的距离。此迭代器等于 std::default_sentinel 当且仅当其计数抵达零。

成员类型

成员类型	定义
iterator_type	I
value_type	若 I 实现 indirectly_readable 则为 std::iter_value_t<I> ；否则不定义
difference_type	std::iter_difference_t<I>
iterator_concept	若 I::iterator_concept 存在则为它；否则不定义
iterator_category	若 I::iterator_category 存在则为它；否则不定义

成员函数

(构造函数) (C++20)	构造新的迭代器适配器 (公开成员函数)
operator= (C++20)	赋值另一迭代器适配器 (公开成员函数)
base (C++20)	访问底层迭代器 (公开成员函数)
count (C++20)	返回到末尾的距离 (公开成员函数)
operator*operator-> (C++20)	访问被指向的元素 (公开成员函数)
operator[] (C++20)	按索引访问元素 (公开成员函数)
operator++operator++(int)operator+=operator+operator--operator--(int)operator-=operator- (C++20)	推进或回退迭代器 (公开成员函数)

成员对象

成员名	定义
current (私有)	base() 所访问的底层迭代器，名字仅用于阐释
length (私有)	底层迭代器与其范围末尾的距离，名字仅用于阐释

非成员函数

operator==operator<=> (C++20)	比较到末尾的距离 (函数模板)
operator==(std::default_sentinel) (C++20)	检查到末尾的距离是否等于 ​0​ (函数模板)
operator+ (C++20)	令迭代器前进 (函数模板)
operator- (C++20)	计算两个迭代器适配器间的距离 (函数模板)
operator-(std::default_sentinel_t) (C++20)	计算到末尾的有符号距离 (函数模板)
iter_move (C++20)	转型解引用底层迭代器的结果为其所关联的右值引用类型 (函数)
iter_swap (C++20)	交换二个底层迭代器所指向的对象 (函数模板)

辅助类

std::iterator_traits<std::counted_iterator> (C++20)

对 std::counted_iterator 类型的属性提供均一的接口 (类模板特化)

示例

运行此代码

#include <algorithm>
#include <iostream>
#include <iterator>
#include <string>
#include <vector>
using std::operator""s;
 
void print(auto const remark, auto const& v) {
    const auto size = std::ssize(v);
    std::cout << remark << "[" << size << "] { ";
    for (auto it = std::counted_iterator{std::cbegin(v), size};
         it != std::default_sentinel; ++it) {
        std::cout << *it << ", ";
    }
    std::cout << "}\n";
}
 
int main() {
    const auto src = {"Arcturus"s, "Betelgeuse"s, "Canopus"s, "Deneb"s, "Elnath"s };
    print("src", src);
    std::vector<decltype(src)::value_type> dst;
    std::ranges::copy(std::counted_iterator{src.begin(), 3},
                      std::default_sentinel,
                      std::back_inserter(dst));
    print("dst", dst);
}

输出：

src[5] { Arcturus, Betelgeuse, Canopus, Deneb, Elnath, }
dst[3] { Arcturus, Betelgeuse, Canopus, }

缺陷报告

下列更改行为的缺陷报告追溯地应用于以前出版的 C++ 标准。

DR	应用于	出版时的行为	正确行为
P2259R1	C++20	不提供成员 typedef 对 counted_iterator 特化 std::incrementable_traits	添加成员 typedef 以顾及 iterator_traits 修正 移除了冗余的 std::incrementable_traits 特化

参阅

default_sentinel_t (C++20)	用于知晓其边界的迭代器的默认哨位 (类)
views::counted (C++20)	从迭代器和计数创建子范围 (定制点对象)
ranges::take_viewviews::take (C++20)	由另一 view 的前 N 个元素组成的 view (类模板) (范围适配器对象)