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.

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).



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.


Following derived types may use the default implementation of empty:

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.


#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.

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

See also

checks whether the container is empty
(function template)
checks whether a range is empty
(customization point object)

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