# std::ranges::dangling

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< cpp‎ | ranges

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> constexpr dangling(Args&&...) noexcept { } | (2) |

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

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