operator==,<=>(std::counted_iterator)

 

C++

语言

标准库头文件

自立与有宿主实现

具名要求

语言支持库

概念库 (C++20)

诊断库

工具库

字符串库

容器库

迭代器库

范围库 (C++20)

算法库

数值库

本地化库

输入/输出库

文件系统库 (C++17)

正则表达式库 (C++11)

原子操作库 (C++11)

线程支持库 (C++11)

技术规范

 

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)

 

template< std::common_with<I> I2 >   friend constexpr bool operator==(     const counted_iterator& x, const counted_iterator<I2>& y );	(1)	(C++20 起)
template< std::common_with<I> I2 >   friend constexpr strong_ordering operator<=>(     const counted_iterator& x, const counted_iterator<I2>& y );	(2)	(C++20 起)

比较底层长度（即到末尾的距离）。

1) 检查长度是否相等。

2) 用运算符 <=> 比较底层长度。

若 x 与 y 不指向同一序列的元素则行为未定义。即必须存在某个 n 使得 std::next(x.base(), x.count() + n) 与 std::next(y.base(), y.count() + n) 指代同一元素。

<、 <=、 >、 >= 及 != 运算符分别从 operator<=> 与 operator== 合成。

此函数模板对通常无限定或有限定查找不可见，而只能在 std::counted_iterator<I> 为参数的关联类时由实参依赖查找找到。

参数

x, y

-

迭代器适配器

返回值

1) x.count() == y.count()

2) y.count() <=> x.count()

注解

由于长度向下计数，而非向上， operator<=> 的参数在底层比较表达式中的顺序是逆转的，即 y 为 lhs ， x 为 rhs 。

示例

运行此代码

#include <initializer_list>
#include <iterator>
 
int main()
{
    static constexpr auto v = {1, 2, 3, 4, 5, 6};
    constexpr std::counted_iterator<std::initializer_list<int>::iterator>
        it1 {v.begin(), 5},
        it2 {v.begin(), 5},
        it3 {v.begin() + 1, 4},
        it4 {v.begin(), 0};
    static_assert( it1 == it2 );
    static_assert( it2 != it3 );
    static_assert( it2 <  it3 );
    static_assert( it1 <= it2 );
    static_assert( it3 != std::default_sentinel );
    static_assert( it4 == std::default_sentinel );
 
    // it2 == std::counted_iterator{v.begin(), 4}; // UB ：运算数不指代同一序列的元素
}

参阅

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)