Difference between revisions of "cpp/ranges/dangling"
From cppreference.com
m (~) |
m (→See also: += ranges::borrowed_range) |
||
| Line 68: | Line 68: | ||
{{dsc begin}} | {{dsc begin}} | ||
{{dsc inc | cpp/ranges/dsc borrowed_iterator_t}} | {{dsc inc | cpp/ranges/dsc borrowed_iterator_t}} | ||
| + | {{dsc inc | cpp/ranges/dsc borrowed_range}} | ||
{{dsc end}} | {{dsc end}} | ||
| − | {{langlinks|es|ja|zh}} | + | {{langlinks|es|ja|ru|zh}} |
Revision as of 04:45, 24 April 2021
| 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 |
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
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) |
