std::unreachable_sentinel_t, std::unreachable_sentinel

 

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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.

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 <cstddef>
#include <iterator>
#include <algorithm>
#include <iostream>
 
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 pos(const CharT *haystack, const CharT *needle)
{
    // search(begin, unreachable_sentinel) is usually more efficient than
    // search(begin, end) due to one less comparison per cycle.
    // But "needle" MUST BE in the "haystack", otherwise the call is UB,
    // which is a compile-time error in constexpr context.
    return std::ranges::search(
               haystack, std::unreachable_sentinel,
               needle, needle + strlen(needle)
           ).begin() - haystack;
}
 
int main()
{
    static_assert(
        strlen("The quick brown fox jumps over the lazy dog.") == 44
    );
 
    static_assert(
        pos("const short int", "short") == 6
    );
 
//  static_assert(pos("long int", "float")); // compile-time error
}

See also

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

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