NAME

std::unreachable_sentinel_t,std::unreachable_sentinel - std::unreachable_sentinel_t,std::unreachable_sentinel

Synopsis

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

operator==(std::unreachable_sentinel_t)

template<std::weakly_incrementable I>

friend constexpr bool operator==( unreachable_sentinel_t, const I& ) (since C++20)
noexcept

{ return false; }

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