std::unreachable_sentinel_t,std::unreachable_sentinel

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Iterator library

Iterator concepts

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)
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forward_iterator (C++20)
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Iterator primitives

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 (deprecated in C++17)
iterator_traits
incrementable_traits (C++20)
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indirectly_unary_invocableindirectly_regular_unary_invocable

(C++20)(C++20)

indirect_unary_predicate (C++20)
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indirectly_movable (C++20)
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(C++20)

(C++20)

(C++26)

reverse_iterator
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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)

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Range access

begincbegin (C++11)(C++14)
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endcend (C++11)(C++14)
rendcrend (C++14)(C++14)

sizessize (C++17)(C++20)
empty (C++17)
data (C++17)

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Defined in header`<iterator>`
struct unreachable_sentinel_t;	(1)	(since C++20)
inlineconstexpr unreachable_sentinel_t unreachable_sentinel{};	(2)	(since C++20)

1)unreachable_sentinel_t is an empty class type that can be used to denote the “upper bound” of an unbounded interval.

2)unreachable_sentinel is a constant of typeunreachable_sentinel_t.

[edit]Non-member functions

operator==

(C++20)

compares anunreachable_sentinel_t with a value of anyweakly_incrementable type
(function template)

operator==(std::unreachable_sentinel_t)

template<std::weakly_incrementable I> friendconstexprbool operator==( unreachable_sentinel_t,const I&)noexcept {returnfalse;}		(since C++20)

unreachable_sentinel_t can be compared with anyweakly_incrementable type and the result is alwaysfalse.

This function template is not visible to ordinaryunqualified orqualified lookup, and can only be found byargument-dependent lookup whenstd::unreachable_sentinel_t is an associated class of the arguments.

[edit]Example

Run this code

#include <concepts>#include <cstddef>#include <iterator>#include <ranges>#include <utility> namespace ranges= std::ranges; // never checks “iter != r.end()”template<ranges::random_access_range R>constexprstd::size_t trivial_strlen(R&& r){auto iter= r.begin();while(*iter!=ranges::range_value_t<R>{})++iter;return iter- r.begin();} template<ranges::random_access_range R>constexprstd::size_t my_strlen(R&& r){ifconstexpr(std::same_as<ranges::sentinel_t<R>,                               std::unreachable_sentinel_t>)return trivial_strlen(std::forward<R>(r));elsereturnranges::find(std::forward<R>(r),ranges::range_value_t<R>{})-ranges::begin(r);} int main(){constexprstaticchar str[]="The quick brown fox jumps over a lazy dog.";    static_assert(my_strlen(str)==42); // finds the length of the string faster, but UB if “str” is not null-terminatedconstexprauto unsafe_str=ranges::subrange{str, std::unreachable_sentinel};    static_assert(my_strlen(unsafe_str)==42);}