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

< cpp‎ | ranges
Defined in header <ranges>
template< ranges::input_range V, std::size_t N >

        view<V> &&
        __HasTupleElement<ranges::range_value_t<V>, N> &&
        __HasTupleElement<std::remove_reference_t<ranges::range_reference_t<V>>, N>

class elements_view : public ranges::view_interface<elements_view<V, N>>;
(1) (since C++20)
namespace views {

    inline constexpr /*unspecified*/ elements = /*unspecified*/;

(2) (since C++20)
Helper concept
template< class T, std::size_t N >

concept __HasTupleElement =  // exposition only
    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>&>;

(3) (since C++20)
1) Accepts a view of tuple-like values, and issues a view with a value-type of the Nth element of the adapted view's value-type, where N is the non-type template parameter.
2) Range adaptor object. The expression views​::​elements<N>(E) is expression-equivalent to elements_­view<views​::​all_­t<decltype((E))>, N>{E} for any suitable subexpression E and constant expression N.
3) Ensures that the elements of the underlying view are tuple-like values.

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.


[edit] Expression-equivalent

Expression e is expression-equivalent to expression f, if e and f have the same effects, either are both potentially-throwing or are both not potentially-throwing (i.e. noexcept(e) == noexcept(f)), and either are both constant subexpressions or are both not constant subexpressions.

[edit] Member functions

constructs a elements_view
(public member function) [edit]
returns a copy of the underlying (adapted) view
(public member function) [edit]
returns an iterator to the beginning
(public member function) [edit]
returns an iterator or a sentinel to the end
(public member function) [edit]
returns the number of elements. Provided only if the underlying (adapted) range satisfies sized_range
(public member function) [edit]

[edit] Deduction guides

[edit] Nested classes

the iterator type
(public member class)
the sentinel type
(public member class)

[edit] Helper templates

template<class T, std::size_t N>

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

(since C++20)

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

[edit] Example

#include <iostream>
#include <ranges>
#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: std::views::elements<1>(vt)) { std::cout << e << ' '; }
    std::cout << '\n';
    for (std::string const& e: std::views::elements<2>(vt)) { std::cout << e << ' '; }
    std::cout << '\n';


1 2 3 4 5 
A B C D E 
α β γ δ ε

[edit] 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

[edit] See also

takes a view consisting of pair-like values and produces a view of the first elements of each pair
(class template) (range adaptor object) [edit]
takes a view consisting of pair-like values and produces a view of the second elements of each pair
(class template) (range adaptor object) [edit]
BLAS-like slice of a valarray: starting index, length, stride
(class) [edit]