NAME¶

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

Synopsis¶

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< (since
(1) C++20)
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>>;
namespace views {

template< std::size_t N > (since
inline constexpr /* unspecified */ elements = /* (2) C++20)
unspecified */;

}
Call signature
template< ranges::viewable_range R >
(since
requires /* see below */ C++20)

constexpr ranges::view auto elements<N>( R&& r );
Helper concepts
template< class T, std::size_t N >

concept has-tuple-element =
requires(T t) {
typename std::tuple_size<T>::type; (until C++23)
requires N < std::tuple_size_v<T>; (exposition
typename std::tuple_element_t<N, T>; only*)
{ std::get<N>(t) } -> std::convertible_to<
const
std::tuple_element_t<N, T>&>;

}; (3)
template< class T, std::size_t N >
(since C++23)
concept has-tuple-element = (exposition
only*)
tuple-like<T> && N < std::tuple_size_v<T>
template< class T, std::size_t N >

concept returnable-element = (exposition
std::is_reference_v<T> || std::move_constructible< (4) only*)

std::tuple_element_t<N, T>>;

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¶

constructor constructs a elements_view
(C++20) (public member function)
base returns a copy of the underlying (adapted) view
(C++20) (public member function)
begin returns an iterator to the beginning
(C++20) (public member function)
end returns an iterator or a sentinel to the end
(C++20) (public member function)
size returns the number of elements. Provided only if the underlying
(C++20) (adapted) range satisfies sized_range.
(public member function)
Inherited from std::ranges::view_interface
empty returns whether the derived view is empty. Provided if it satisfies
(C++20) sized_range or forward_range.
(public member function of std::ranges::view_interface<D>)
cbegin returns a constant iterator to the beginning of the range.
(C++23) (public member function of std::ranges::view_interface<D>)
cend returns a sentinel for the constant iterator of the range.
(C++23) (public member function of std::ranges::view_interface<D>)
operator bool returns whether the derived view is not empty. Provided if
(C++20) ranges::empty is applicable to it.
(public member function of std::ranges::view_interface<D>)
front returns the first element in the derived view. Provided if it
(C++20) satisfies forward_range.
(public member function of std::ranges::view_interface<D>)
back returns the last element in the derived view. Provided if it satisfies
(C++20) bidirectional_range and common_range.
(public member function of std::ranges::view_interface<D>)
operator[] returns the n^th element in the derived view. Provided if it satisfies
(C++20) random_access_range.
(public member function of std::ranges::view_interface<D>)

Nested classes

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

Helper templates

template<class T, std::size_t N>

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

std::ranges::enable_borrowed_range<T>;

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

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 it is a borrowed_range
borrowed_range if its underlying view is
LWG 3502 C++20 dangling reference could be obtained such usage is forbidden
from elements_view

See also¶

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