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

Input/output library

Localizations library

Regular expressions library (C++11)

Atomic operations library (C++11)

Thread support library (C++11)

Filesystem library (C++17)

Technical Specifications

 

Ranges library

Range access
begincbegin endcend	rbegincrbegin rendcrend	size empty datacdata
Range primitives
iterator_tsentinel_t	range_difference_trange_value_t	range_reference_trange_rvalue_reference_t
Dangling iterator handling
dangling	safe_iterator_t	safe_subrange_t
Range concepts
range sized_range view	input_range output_range forward_range	bidirectional_range random_access_range contiguous_range	common_range viewable_range
Views
view_interface	subrange
Semiregular wrapper
Factories
empty_viewviews::empty	single_viewviews::single	iota_viewviews::iota	views::counted
Adaptors
all_viewviews::all join_viewviews::join take_viewviews::take basic_istream_viewistream_view	ref_view                                split_viewviews::split take_while_viewviews::take_while elements_viewviews::elements	common_viewviews::common transform_viewviews::transform drop_viewviews::drop keys_viewviews::keys	reverse_viewviews::reverse filter_viewviews::filter drop_while_viewviews::drop_while values_viewviews::values

 

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 adaptor that produces a view of no elements of a particular type.

2) Variable template for empty_view.

Member functions

std::ranges::empty_view::begin

static constexpr T* begin() noexcept { return nullptr; }

empty_view does not reference any element.

std::ranges::empty_view::end

static constexpr T* end() noexcept { return nullptr; }

empty_view does not reference any element.

std::ranges::empty_view::data

static constexpr T* data() noexcept { return nullptr; }

empty_view does not reference any element.

std::ranges::empty_view::size

static constexpr std::size_t size() noexcept { return 0; }

empty_view is always empty.

std::ranges::empty_view::empty

static constexpr bool empty() noexcept { return true; }

empty_view is always empty.

Non-member functions

begin, end(std::ranges::empty_view)

friend constexpr T* begin(empty_view) noexcept { return nullptr; }	(1)
friend constexpr T* end(empty_view) noexcept { return nullptr; }	(2)

These functions are not visible to ordinary unqualified or qualified lookup, and can only be found by argument-dependent lookup when std::ranges::empty_view<T> is an associated class of the arguments.

Example

Run this code

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