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

 

C++

Compiler support

Freestanding and hosted

Language

Standard library

Standard library headers

Named requirements

Feature test macros (C++20)

Language support library

Concepts library (C++20)

Metaprogramming library (C++11)

Diagnostics library

General utilities library

Strings library

Containers library

Iterators library

Ranges library (C++20)

Algorithms library

Numerics library

Localizations library

Input/output library

Filesystem library (C++17)

Regular expressions library (C++11)

Concurrency support library (C++11)

Execution support library (C++26)

Technical specifications

Symbols index

External libraries

 

Ranges library

Range access begin cbegin   end cend rbegin crbegin   rend crend size ssize   data cdata empty

Range conversions std::from_range_t std::from_range (C++23)(C++23)     to (C++23)

Dangling iterator handling dangling borrowed_iterator_t borrowed_subrange_t

Range primitives
range_size_trange_difference_trange_value_t elements_of (C++23)	iterator_tconst_iterator_tsentinel_tconst_sentinel_t (C++23)(C++23)	range_reference_trange_const_reference_trange_rvalue_reference_trange_common_reference_t (C++23)

Range concepts
range borrowed_range    sized_range	common_range view viewable_range	input_range output_range forward_range	bidirectional_range    random_access_range contiguous_range	constant_range (C++23)

Views
view_interface	subrange

Range factories
empty_viewviews::empty	single_viewviews::single	basic_istream_viewviews::istream	iota_viewviews::iota	repeat_viewviews::repeat (C++23)(C++23)

Range adaptors
views::all_tviews::all ref_view owning_view as_rvalue_viewviews::as_rvalue (C++23)(C++23)   filter_viewviews::filter transform_viewviews::transform take_viewviews::take take_while_viewviews::take_while concat_viewviews::concat (C++26)(C++26)	drop_viewviews::drop drop_while_viewviews::drop_while join_viewviews::join join_with_viewviews::join_with (C++23)(C++23)   lazy_split_viewviews::lazy_split split_viewviews::split views::counted common_viewviews::common	reverse_viewviews::reverse as_const_viewviews::as_const (C++23)(C++23) elements_viewviews::elements keys_viewviews::keys values_viewviews::values enumerate_viewviews::enumerate (C++23)(C++23) zip_viewviews::zip (C++23)(C++23) zip_transform_viewviews::zip_transform (C++23)(C++23)	adjacent_viewviews::adjacent (C++23)(C++23) views::pairwise (C++23) adjacent_transform_viewviews::adjacent_transform (C++23)(C++23) views::pairwise_transform (C++23) chunk_viewviews::chunk (C++23)(C++23) slide_viewviews::slide (C++23)(C++23) chunk_by_viewviews::chunk_by (C++23)(C++23) stride_viewviews::stride (C++23)(C++23) cartesian_product_viewviews::cartesian_product (C++23)(C++23)

Range generators std::generator (C++23)

Range adaptor closure objects range_adaptor_closure (C++23)

Range adaptor objects

Helper items
copyable-box movable-box (until C++23)  (C++23)	simple-view non-propagating-cache

 

std::ranges::view_interface

Member functions

empty

cbegin (C++23)

cend (C++23)

operator bool

data

size

front

back

operator[]

 

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).

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 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

(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.

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 forward_range types	sized_range-only types are also supported

See also

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