std::ranges::dangling - cppreference.com (original) (raw)
| | | | | ---------------- | | ------------- | | 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.
Contents
[edit] Member functions
std::ranges::dangling::dangling
| constexpr dangling() noexcept = default; | (1) | | | ------------------------------------------------------------------- | --- | | | template<class... Args> constexpr dangling(Args&&...) noexcept { } | (2) | |
danglingis trivially default constructible.danglingcan 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
#include
#include
#include
#include
#include
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::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::ranges::dangling, decltype(valid_iter2)>);
std::cout << ''' << *valid_iter2 << ''' << '\n'; // 'a'
}
Output: