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std::ranges::views::elements, std::ranges::elements_view

Defined in header <ranges> 
template< ranges::input_range V, std::size_t N >
    requires ranges::view<V> &&
             has-tuple-element<ranges::range_value_t<V>, N> &&
             has-tuple-element<std::remove_reference_t<
                                   ranges::range_reference_t<V>>, N> &&
             returnable-element<ranges::range_reference_t<V>, N>
class elements_view : public ranges::view_interface<elements_view<V, N>>;
 (1) (since C++20) 
namespace views {
    template< std::size_t N >
    inline constexpr /* unspecified */ elements = /* unspecified */;
}
 (2) (since C++20)
Call signature 
template< ranges::viewable_range R >
    requires /* see below */
constexpr ranges::view auto elements<N>( R&& r );
 (since C++20)
Helper concepts
(3) 
template< class T, std::size_t N >
concept has-tuple-element =
    requires(T t) {
        typename std::tuple_size<T>::type;
        requires N < std::tuple_size_v<T>;
        typename std::tuple_element_t<N, T>;
        { std::get<N>(t) } -> std::convertible_to<
                                  const std::tuple_element_t<N, T>&>;
    };
 (until C++23)
(exposition only*)		 
template< class T, std::size_t N >
concept has-tuple-element =
    tuple-like<T> && N < std::tuple_size_v<T>
 (since C++23)
(exposition only*)		 
template< class T, std::size_t N >
concept returnable-element =
     std::is_reference_v<T> || std::move_constructible<
                                       std::tuple_element_t<N, T>>;
 (4) (exposition only*)
1) Accepts a view of tuple-like values, and issues a view with a value type of the N-th element of the adapted view's value-type.
2) Every specialization of views::elements is a RangeAdaptorObject. The expression views::elements<M>(e) is expression-equivalent to elements_view<views::all_t<decltype((e))>, M>{e} for any suitable subexpression e and constant expression M.
3) Ensures that the elements of the underlying view are tuple-like values, see tuple-like(since C++23).
4) Ensures that dangling references cannot be returned.

elements_view models the concepts random_access_range, bidirectional_range, forward_range, input_range, common_range, and sized_range when the underlying view V models respective concepts.

Member functions

(C++20)
constructs a elements_view
(public member function)
(C++20)
returns a copy of the underlying (adapted) view
(public member function)
(C++20)
returns an iterator to the beginning
(public member function)
(C++20)
returns an iterator or a sentinel to the end
(public member function)
(C++20)
returns the number of elements. Provided only if the underlying (adapted) range satisfies sized_range.
(public member function)
Inherited from std::ranges::view_interface
(C++20)
returns whether the derived view is empty. Provided if it satisfies sized_range or forward_range.
(public member function of std::ranges::view_interface<D>)
(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>)

Nested classes

(C++20)
the iterator type
(exposition-only member class template*)
(C++20)
the sentinel type
(exposition-only member class template*)

Helper templates

template<class T, std::size_t N>
inline constexpr bool enable_borrowed_range<std::ranges::elements_view<T, N>> =
    std::ranges::enable_borrowed_range<T>;
 (since C++20)

This specialization of std::ranges::enable_borrowed_range makes elements_view satisfy borrowed_range when the underlying view satisfies it.

Example

#include <iostream>
#include <ranges>
#include <string>
#include <tuple>
#include <vector>
 
int main()
{
    const std::vector<std::tuple<int, char, std::string>> vt
    {
        {1, 'A', "α"},
        {2, 'B', "β"},
        {3, 'C', "γ"},
        {4, 'D', "δ"},
        {5, 'E', "ε"},
    };
 
    for (int const e : std::views::elements<0>(vt))
        std::cout << e << ' ';
    std::cout << '\n';
 
    for (char const e : vt | std::views::elements<1>)
        std::cout << e << ' ';
    std::cout << '\n';
 
    for (std::string const& e : std::views::elements<2>(vt))
        std::cout << e << ' ';
    std::cout << '\n';
}

Output:

1 2 3 4 5
A B C D E
α β γ δ ε

Defect reports

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

DR Applied to Behavior as published Correct behavior
LWG 3494 C++20 elements_view was never a borrowed_range it is a borrowed_range
if its underlying view is
LWG 3502 C++20 dangling reference could be obtained from elements_view such usage is forbidden

See also

(C++20)
takes a view consisting of pair-like values and produces a view of the first elements of each pair
(class template) (range adaptor object)
(C++20)
takes a view consisting of pair-like values and produces a view of the second elements of each pair
(class template) (range adaptor object)
(C++23)
a view consisting of tuples of references to corresponding elements of the adapted views
(class template) (customization point object)
(C++23)
a view consisting of tuples of results of application of a transformation function to corresponding elements of the adapted views
(class template) (customization point object)
BLAS-like slice of a valarray: starting index, length, stride
(class)

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