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

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
(C++23)
returns a constant iterator to the beginning of the range.
(public member function of std::ranges::view_interface<D>)
(C++23)
returns a sentinel for the constant iterator of the range.
(public member function of std::ranges::view_interface<D>)
(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>)
(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>)
(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>)
(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

#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

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

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https://en.cppreference.com/w/cpp/ranges/empty_view