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

 

C++

Compiler support

Freestanding and hosted

Language

Standard library headers

Named requirements

Feature test macros (C++20)

Language support library

Concepts library (C++20)

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)

Atomic operations library (C++11)

Thread support library (C++11)

Technical specifications

Symbols index

External libraries

 

Ranges library

Range access
begin cbegin end cend	rbegin crbegin rend crend	size ssize data cdata	empty
Range primitives
iterator_tsentinel_t	range_size_trange_difference_t	range_reference_trange_rvalue_reference_t	range_value_t
Dangling iterator handling
dangling	borrowed_iterator_t	borrowed_subrange_t
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
Views
view_interface	subrange
Factories
empty_viewviews::empty	single_viewviews::single	iota_viewviews::iota	basic_istream_viewviews::istream
Adaptors
views::all_tviews::all ref_view owning_view common_viewviews::common reverse_viewviews::reverse views::counted zip_viewviews::zip (C++23)(C++23)	take_viewviews::take take_while_viewviews::take_while drop_viewviews::drop drop_while_viewviews::drop_while zip_transform_viewviews::zip_transform (C++23)(C++23)	elements_viewviews::elements keys_viewviews::keys values_viewviews::values transform_viewviews::transform adjacent_viewviews::adjacent (C++23)(C++23)	filter_viewviews::filter join_viewviews::join split_viewviews::split lazy_split_viewviews::lazy_split adjacent_transform_viewviews::adjacent_transform (C++23)(C++23)
Range adaptor objects
Range adaptor closure objects
Helper items
copyable-box	simple-view

 

std::ranges::view_interface

Member functions

empty (C++20)

operator bool (C++20)

data (C++20)

size (C++20)

front (C++20)

back (C++20)

operator[] (C++20)

 

constexpr bool empty() requires ranges::forward_range<D>;	(1)	(since C++20)
constexpr bool empty() const requires ranges::forward_range<const D>;	(2)	(since C++20)

The default implementation of empty() member function checks 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::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 beginning iterator and the sentinel of the object of the derived type 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::iota_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

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 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(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() );
}

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)