Ranges library
Range access
Range conversions
Range primitives

Dangling iterator handling
Range concepts

Range generators
Range adaptor objects
Range adaptor closure objects
Helper items
Member functions
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).


[edit] Parameters


[edit] 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.

[edit] Notes

Following derived types may use the default implementation of empty:

(since C++23)

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.

[edit] 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() );

[edit] 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

[edit] See also

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