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

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< cpp‎ | ranges
C++
Ranges library
Range adaptors
std::ranges::elements_view
Member functions
Nested classes
Iterator
Sentinel
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 >
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
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

Data members

Member name Definition
base_ (private) the underlying (adapted) view of type V
(exposition-only member object*

Member functions

 constructs a elements_view
(public member function)
returns a copy of the underlying (adapted) view
(public member function)
returns an iterator to the beginning
(public member function)
returns an iterator or a sentinel to the end
(public member function)
returns the number of elements. Provided only if the underlying (adapted) range satisfies sized_range.
(public member function)
Inherited from std::ranges::view_interface
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>)
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>)
returns the first element in the derived view. Provided if it satisfies forward_range.
(public member function of std::ranges::view_interface<D>)
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>)
returns the n th element in the derived view. Provided if it satisfies random_access_range.
(public member function of std::ranges::view_interface<D>)

Nested classes

 the iterator type
(exposition-only member class template*
the sentinel type
(exposition-only member class template*

Helper templates

template < class T, std::size_t N>

constexpr bool enable_borrowed_range<std:: ranges :: elements_view <T, N>> =

ranges::enable_borrowed_range <T> ;
(since C++20)

This specialization of ranges::enable_borrowed_range makes elements_view satisfy borrowed_range

Example

Run this code
#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 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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