The default implementation ofempty() 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.

[edit]Return value

true if the size of the object of the derived type is​0​ (ifD satisfiesstd::ranges::sized_range), or its beginning iterator and the sentinel compare equal,false otherwise.

[edit]Notes

Following derived types may use the default implementation ofempty:

• 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

Althoughstd::ranges::basic_istream_view inherits fromstd::ranges::view_interface and does not declare theempty() member function, it cannot use the default implementation, because it never satisfies neitherstd::ranges::sized_range norstd::ranges::forward_range.

[edit]Example

#include <array>#include <ranges> int main(){constexprstd::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.

[edit]See also

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