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.
constexpr dangling() noexcept = default; | (1) | |
template<class... Args>
constexpr dangling(Args&&...) noexcept { }
| (2) |
dangling is trivially default constructible.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.
#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'
|
(C++20) | obtains iterator type or subrange type of a borrowed_range (alias template) |
|
(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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https://en.cppreference.com/w/cpp/ranges/dangling