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std::ranges::views::empty, std::ranges::empty_view - cppreference.com

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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>
    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]

 returns nullptr
(public static member function)

end

[static]

 returns nullptr
(public static member function)

data

[static]

 returns nullptr
(public static member function)

size

[static]

 returns 0
(public static member function)

empty

[static]

 returns true
(public static member function)
Inherited from ranges::view_interface

(C++23)

 returns a constant iterator to the beginning of the range
(public member function of std::ranges::view_interface<D>) [edit]

(C++23)

 returns a sentinel for the constant iterator of the range
(public member function of std::ranges::view_interface<D>) [edit]
returns whether the derived view is not empty, provided only if ranges::empty is applicable to it
(public member function of std::ranges::view_interface<D>) [edit]
returns the first element in the derived view, provided if it satisfies forward_range
(public member function of std::ranges::view_interface<D>) [edit]
returns the last element in the derived view, provided only if it satisfies bidirectional_range and common_range
(public member function of std::ranges::view_interface<D>) [edit]
returns the nth element in the derived view, provided only if it satisfies random_access_range
(public member function of std::ranges::view_interface<D>) [edit]

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

This specialization of 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

#include <ranges>

int main()
{
    namespace ranges = std::ranges;

    ranges::empty_view<long> e;
    static_assert(ranges::empty(e)); // uses operator bool
    static_assert(0 == e.size());
    static_assert(nullptr == e.data());
    static_assert(nullptr == e.begin());
    static_assert(nullptr == e.end());
    static_assert(nullptr == e.cbegin());
    static_assert(nullptr == e.cend());
}

See also