std::ranges::dangling(3)

NAME
std::ranges::dangling - std::ranges::dangling

Synopsis
   Defined in header <ranges>
   struct dangling; (since C++20)
   dangling is a placeholder type and an empty class type, used together with the
   template aliases ranges::borrowed_iterator_t and ranges::borrowed_subrange_t.
   When some constrained algorithms that usually return an iterator or a subrange of a
   range take a particular rvalue range argument that does not model borrowed_range,
   dangling will be returned instead to avoid returning potentially dangling results.

Member functions
std::ranges::dangling::dangling
   constexpr dangling() noexcept = default; (1)
   template<class... Args> (2)
   constexpr dangling(Args&&...) noexcept { }
   1) dangling is trivially default constructible.
   2) dangling can be constructed from arguments of arbitrary number and arbitrary
   non-void type. The construction does not have any side-effect itself.
   In other words, after replacing the type (e.g. an iterator type) in a well-formed
   non-aggregate initialization with dangling, the resulting initialization is also
   well-formed.

Example
// Run this code
 #include <algorithm>
 #include <array>
 #include <iostream>
 #include <ranges>
 #include <type_traits>
 #include <string_view>
 int main()
 {
     auto get_array_by_value = [] { return std::array{0, 1, 0, 1}; };
     auto dangling_iter = std::ranges::max_element(get_array_by_value());
     static_assert(std::is_same_v<std::ranges::dangling, decltype(dangling_iter)>);
 // std::cout << *dangling_iter << '\n'; // compilation error: no match for 'operator*'
                        // (operand type is 'std::ranges::dangling')
     auto get_persistent_array = []() -> const std::array<int, 4>& {
         static constexpr std::array a{0, 1, 0, 1};
         return a;
     };
     auto valid_iter = std::ranges::max_element(get_persistent_array());
     static_assert(!std::is_same_v<std::ranges::dangling, decltype(valid_iter)>);
     std::cout << *valid_iter << ' '; // 1
     auto get_string_view = [] { return std::string_view{"alpha"}; };
     auto valid_iter2 = std::ranges::min_element(get_string_view());
         // OK: std::basic_string_view models borrowed_range
     static_assert(!std::is_same_v<std::ranges::dangling, decltype(valid_iter2)>);
     std::cout << '\'' << *valid_iter2 << '\'' << '\n'; // 'a'
 }

Output:
 1 'a'

See also
   ranges::borrowed_iterator_t obtains iterator type or subrange type of a
   ranges::borrowed_subrange_t borrowed_range
   (C++20) (alias template)
                        specifies that a type is a range and iterators obtained
   ranges::borrowed_range from an expression of it can be safely returned without
   (C++20) danger of dangling
                        (concept)