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Variants

std:: unreachable_sentinel_t, std:: unreachable_sentinel

From cppreference.net
C++
Iterator library
Iterator concepts
Iterator primitives
Algorithm concepts and utilities
Indirect callable concepts
Common algorithm requirements
(C++20)
(C++20)
(C++20)
Utilities
(C++20)
Iterator adaptors
(C++14)
(C++11)
(C++11)
(C++20) (C++20)
unreachable_sentinel_t unreachable_sentinel
(C++20) (C++20)
Range access
(C++11) (C++14)
(C++14) (C++14)
(C++11) (C++14)
(C++14) (C++14)
(C++17) (C++20)
(C++17)
(C++17)
Defined in header <iterator>
struct unreachable_sentinel_t ;
(1) (since C++20)
inline constexpr 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 type unreachable_sentinel_t .

Contents

Non-member functions

operator==
(C++20)
compares an unreachable_sentinel_t with a value of any weakly_incrementable type
(function template)

operator== (std::unreachable_sentinel_t)

template < std:: weakly_incrementable I >

friend constexpr bool operator == ( unreachable_sentinel_t, const I & ) noexcept

{ return false ; }
(since C++20)

unreachable_sentinel_t can be compared with any weakly_incrementable type and the result is always false .

This function template is not visible to ordinary unqualified or qualified lookup , and can only be found by argument-dependent lookup when std::unreachable_sentinel_t is an associated class of the arguments.

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>
constexpr std::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>
constexpr std::size_t my_strlen(R&& r)
{
    if constexpr (std::same_as<ranges::sentinel_t<R>,
                               std::unreachable_sentinel_t>)
        return trivial_strlen(std::forward<R>(r));
    else
        return ranges::find(std::forward<R>(r), ranges::range_value_t<R>{}) -
               ranges::begin(r);
}
int main()
{
    constexpr static char 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-terminated
    constexpr auto unsafe_str = ranges::subrange{str, std::unreachable_sentinel};
    static_assert(my_strlen(unsafe_str) == 42);
}

See also

(C++20)
a view consisting of a sequence generated by repeatedly incrementing an initial value
(class template) (customization point object)