std::ranges::view_interface::empty(3)

NAME
std::ranges::view_interface::empty - std::ranges::view_interface::empty

Synopsis
   constexpr bool empty() (1) (since C++20)
       requires ranges::sized_range<D> || ranges::forward_range<D>;
   constexpr bool empty() const
       requires ranges::sized_range<const D> || (2) (since C++20)
   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.
   1) Let derived be a reference bound to static_cast<D&>(*this). Equivalent to return
   ranges::size(derived) == 0; if D satisfies sized_range. Otherwise, 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 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::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
     * std::ranges::adjacent_transform_view
     * std::ranges::adjacent_view
     * std::ranges::as_const_view
     * std::ranges::as_rvalue_view
     * std::ranges::cartesian_product_view
     * std::ranges::chunk_view
     * std::ranges::chunk_by_view (since C++23)
     * std::ranges::join_with_view
     * std::ranges::repeat_view
     * std::ranges::slide_view
     * std::ranges::stride_view
     * std::ranges::zip_view
     * std::ranges::zip_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
// Run this code
 #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());
 }
   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 sized_range-only types are also
                       forward_range types supported

See also
   empty checks whether the container is empty
   (C++17) (function template)
   ranges::empty checks whether a range is empty
   (C++20) (customization point object)