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std::ranges::dangling

From cppreference.com
< 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 models borrowed_range, dangling will be returned instead to avoid returning potentially dangling results.

Contents

[edit] 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.

[edit] Example

#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'

[edit] See also

obtains iterator type or subrange type of a borrowed_range
(alias template) [edit]