std::ranges::view_interface<D>::empty
constexpr bool empty() requires ranges::forward_range<D>; |
(1) | (since C++20) |
constexpr bool empty() const requires ranges::forward_range<const D>; |
(2) | (since C++20) |
The default implementation of empty() member function checks whether the beginning iterator and the sentinel compare equal.
1) Let
derived be a reference bound to
static_cast<D&>(*this). Equivalent to
return ranges::begin(derived) == ranges::end(derived);.
2) Same as
(1), except that
derived is
static_cast<const D&>(*this).
Parameters
(none).
Return value
true if the beginning iterator and the sentinel of the object of the derived type compare equal, false otherwise.
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::iota_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 std::ranges::forward_range.
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(2)).empty() );
static_assert( (a | std::views::drop(9)).empty() );
static_assert( ! (a | std::views::drop(2)).empty() );
static_assert( std::views::iota(0,0).empty() );
static_assert( ! std::views::iota(0).empty() );
}
See also
|
(C++17)
|
checks whether the container is empty (function template) |
|
(C++20)
|
checks whether a range is empty (customization point object) |
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