std::unreachable_sentinel_t, std::unreachable_sentinel

 

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indirectly_readable (C++20) indirectly_writable (C++20) weakly_incrementable (C++20) incrementable (C++20) is-integer-like is-signed-integer-like (C++20)(C++20)	sentinel_for (C++20) sized_sentinel_for (C++20) input_iterator (C++20) output_iterator (C++20) input_or_output_iterator (C++20)	forward_iterator (C++20) bidirectional_iterator (C++20) random_access_iterator (C++20) contiguous_iterator (C++20)
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reverse_iterator make_reverse_iterator (C++14) move_iterator (C++11) make_move_iterator (C++11) default_sentinel_tdefault_sentinel (C++20)(C++20) unreachable_sentinel_tunreachable_sentinel (C++20)(C++20)	front_insert_iterator back_insert_iterator inserter insert_iterator front_inserter back_inserter move_sentinel (C++20)	common_iterator (C++20) counted_iterator (C++20) basic_const_iterator (C++23) const_iterator (C++23) const_sentinel (C++23) make_const_iterator (C++23) make_const_sentinel (C++23)
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定义于头文件 <iterator>
struct unreachable_sentinel_t;	(1)	(自 C++20 起)
inline constexpr unreachable_sentinel_t unreachable_sentinel{};	(2)	(自 C++20 起)

1) unreachable_sentinel_t 是一个空类类型，可用于表示无界区间的“上界”。

2) unreachable_sentinel 是 unreachable_sentinel_t 类型的常量。

非成员函数

operator== (C++20)

将 unreachable_sentinel_t 与任何 weakly_incrementable 类型的值进行比较 (函数模板)

operator==(std::unreachable_sentinel_t)

template<std::weakly_incrementable I> friend constexpr bool operator==( unreachable_sentinel_t, const I& ) noexcept { return false; }		(自 C++20 起)

unreachable_sentinel_t 可以与任何 weakly_incrementable 类型比较，结果始终为 false。

此函数模板对于普通的非限定或限定查找不可见，只能通过当 std::unreachable_sentinel_t 是参数的关联类时的实参依赖查找找到。

示例

运行此代码

#include <algorithm>
#include <cstddef>
#include <iostream>
#include <iterator>
 
template<class CharT>
constexpr std::size_t strlen(const CharT* s)
{
    return std::ranges::find(s, std::unreachable_sentinel, CharT{}) - s;
}
 
template<class CharT>
constexpr std::size_t find_first(const CharT* haystack, const CharT* needle)
{
    const char* needle_end = needle + strlen(needle);
    // search(begin, unreachable_sentinel) is usually more efficient than
    // search(begin, end) due to one less comparison per cycle.
    // But "needle" MUST BE PRESENT in the "haystack", otherwise the call
    // is UB (which is a compile-time error in constexpr context).
    auto found = std::ranges::search(haystack, std::unreachable_sentinel,
                                     needle, needle_end);
    return found.begin() - haystack;
}
 
int main()
{
    static_assert(strlen("The quick brown fox jumps over a lazy dog.") == 42);
    static_assert(find_first("unsigned short int", "short") == 9);
//  static_assert(find_first("long int", "float")); // compile-time error
}

参见

ranges::iota_viewviews::iota (C++20)

一个由重复递增初始值生成的序列组成的 view (类模板) (定制点对象)