std::ranges::view_interface<D>::empty
constexpr bool empty() requires ranges::sized_range<D> || ranges::forward_range<D>; |
(1) | (since C++20) |
constexpr bool empty() const requires ranges::sized_range<const D> || ranges::forward_range<const D>; |
(2) | (since C++20) |
The default implementation of empty()
member function checks whether the object of the derived type's size is 0 (if valid), or whether the beginning iterator and the sentinel compare equal.
D
satisfies sized_range
. Otherwise, equivalent to return ranges::begin(derived) == ranges::end(derived);.Contents |
[edit] Parameters
(none)
[edit] Return value
true if the size of the object of the derived type is 0 (if D
satisfies std::ranges::sized_range), or its beginning iterator and the sentinel compare equal, false otherwise.
[edit] Notes
Following derived types may use the default implementation of empty
:
- std::ranges::common_view
- std::ranges::drop_view
- std::ranges::drop_while_view
- std::ranges::elements_view
- std::ranges::filter_view
- std::ranges::join_view
- std::ranges::lazy_split_view
- std::ranges::reverse_view
- std::ranges::single_view
- std::ranges::split_view
- std::ranges::take_view
- std::ranges::take_while_view
- std::ranges::transform_view
Although std::ranges::basic_istream_view inherits from std::ranges::view_interface and does not declare the empty()
member function, it cannot use the default implementation, because it never satisfies neither std::ranges::sized_range nor std::ranges::forward_range.
[edit] Example
#include <array> #include <ranges> int main() { constexpr std::array a{0, 1, 2, 3, 4}; static_assert(!std::ranges::single_view(a).empty()); static_assert((a | std::views::take(0)).empty()); static_assert(!(a | std::views::take(5)).empty()); static_assert((a | std::views::drop(5)).empty()); static_assert(!(a | std::views::drop(3)).empty()); static_assert(std::views::iota(0,0).empty()); static_assert(!std::views::iota(0).empty()); }
[edit] Defect reports
The following behavior-changing defect reports were applied retroactively to previously published C++ standards.
DR | Applied to | Behavior as published | Correct behavior |
---|---|---|---|
LWG 3715 | C++20 | empty() only supported forward_range types
|
sized_range -only types are also supported
|
[edit] See also
(C++17) |
checks whether the container is empty (function template) |
(C++20) |
checks whether a range is empty (customization point object) |