std::ranges::view_interface<D>::empty

constexpr bool empty() 
  requires ranges::sized_range<D> || ranges::forward_range<D>;

constexpr bool empty() const 
  requires ranges::sized_range<const D> || ranges::forward_range<const D>;

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.

Parameters

(none).

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.

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 neither std::ranges::sized_range nor 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() );
}

Defect reports

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

See also