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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input_iterator_tagoutput_iterator_tagforward_iterator_tagbidirectional_iterator_tagrandom_access_iterator_tagcontiguous_iterator_tag (C++20)	iter_value_titer_difference_titer_reference_titer_const_reference_titer_rvalue_reference_titer_common_reference_t (C++20)(C++20)(C++20)(C++23)(C++20)(C++20)	iterator (C++17 中已棄用) iterator_traits incrementable_traits (C++20) indirectly_readable_traits (C++20)
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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)
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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)
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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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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)	範圍訪問 begincbegin (C++11)(C++14) rbegincrbegin (C++14)(C++14)   endcend (C++11)(C++14) rendcrend (C++14)(C++14)   sizessize (C++17)(C++20) empty (C++17) data (C++17)

 

定義於標頭檔案 <iterator>
struct unreachable_sentinel_t;	(1)	(C++20 起)
inline constexpr unreachable_sentinel_t unreachable_sentinel{};	(2)	(C++20 起)

1) unreachable_sentinel_t 是一個空類型別，可用於表示無界區間的“上界”。

2) unreachable_sentinel 是型別 unreachable_sentinel_t 的常量。

非成員函式

operator== (C++20)

將 unreachable_sentinel_t 與任意 weakly_incrementable 型別的值進行比較 (函式模板)

operator==(std::unreachable_sentinel_t)

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

unreachable_sentinel_t 可以與任何 weakly_incrementable 型別進行比較，結果始終為 false。

此函式模板對於普通的非限定查詢或限定查詢不可見，並且僅當 std::unreachable_sentinel_t 是引數的關聯類時，才能透過實參依賴查詢找到它。

示例

執行此程式碼

#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
}

參閱

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

由重複遞增初始值生成的序列組成的 view (類模板) (定製點物件)