std::ranges::views::empty, std::ranges::empty_view

 

C++

Language

Standard Library Headers

Freestanding and hosted implementations

Named requirements

Language support library

Concepts library (C++20)

Diagnostics library

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

 

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_viewistream_view
Adaptors
views::all_tviews::all common_viewviews::common filter_viewviews::filter reverse_viewviews::reverse	take_viewviews::take take_while_viewviews::take_while drop_viewviews::drop drop_while_viewviews::drop_while	elements_viewviews::elements keys_viewviews::keys values_viewviews::values transform_viewviews::transform	join_viewviews::join split_viewviews::split lazy_split_viewviews::lazy_split ref_view views::counted
Range adaptor objects
Range adaptor closure objects
Helper items
copyable-box	simple-view	different-from

 

Defined in header <ranges>
template<class T>     requires std::is_object_v<T> class empty_view : public ranges::view_interface<empty_view<T>>	(1)	(since C++20)
namespace views {     template<class T> inline constexpr empty_view<T> empty{}; }	(2)	(since C++20)

1) A range factory that produces a view of no elements of a particular type.

2) Variable template for empty_view.

Member functions

begin [static] (C++20)	returns nullptr (public static member function)
end [static] (C++20)	returns nullptr (public static member function)
data [static] (C++20)	returns nullptr (public static member function)
size [static] (C++20)	returns ​0​ (zero) (public static member function)
empty [static] (C++20)	returns true (public static member function)
Inherited from std::ranges::view_interface
operator bool (C++20)	Returns whether the derived view is not empty. Provided if ranges::empty is applicable to it. (public member function of std::ranges::view_interface<D>)
front (C++20)	Returns the first element in the derived view. Provided if it satisfies forward_range. (public member function of std::ranges::view_interface<D>)
back (C++20)	Returns the last element in the derived view. Provided if it satisfies bidirectional_range and common_range. (public member function of std::ranges::view_interface<D>)
operator[] (C++20)	Returns the nth element in the derived view. Provided if it satisfies random_access_range. (public member function of std::ranges::view_interface<D>)

std::ranges::empty_view::begin

static constexpr T* begin() noexcept { return nullptr; }		(since C++20)

empty_view does not reference any element.

std::ranges::empty_view::end

static constexpr T* end() noexcept { return nullptr; }		(since C++20)

empty_view does not reference any element.

std::ranges::empty_view::data

static constexpr T* data() noexcept { return nullptr; }		(since C++20)

empty_view does not reference any element.

std::ranges::empty_view::size

static constexpr std::size_t size() noexcept { return 0; }		(since C++20)

empty_view is always empty.

std::ranges::empty_view::empty

static constexpr bool empty() noexcept { return true; }		(since C++20)

empty_view is always empty.

Helper templates

template<class T> inline constexpr bool enable_borrowed_range<ranges::empty_view<T>> = true;		(since C++20)

This specialization of std::ranges::enable_borrowed_range makes empty_view satisfy borrowed_range.

Notes

Although empty_view obtains front, back, and operator[] member functions from view_interface, calls to them always result in undefined behavior since an empty_view is always empty.

The inherited operator bool conversion function always returns false.

Example

Run this code

#include <ranges>
 
int main()
{
    std::ranges::empty_view<long> e;
    static_assert(std::ranges::empty(e));
    static_assert(0 == e.size());
    static_assert(nullptr == e.data());
    static_assert(nullptr == e.begin());
    static_assert(nullptr == e.end());
}

See also

ranges::single_viewviews::single (C++20)	a view that contains a single element of a specified value (class template) (customization point object)
views::all_tviews::all (C++20)	a view that includes all elements of a range (alias template) (range adaptor object)
ranges::ref_view (C++20)	a view of the elements of some other range (class template)