24 Ranges library [ranges]

24.7 Range adaptors [range.adaptors]

24.7.18 Elements view [range.elements]

24.7.18.1 Overview [range.elements.overview]

elements_­view takes a view of tuple-like values and a size_­t, and produces a view with a value-type of the element of the adapted view's value-type.
The name views​::​elements<N> denotes a range adaptor object ([range.adaptor.object]).
Given a subexpression E and constant expression N, the expression views​::​elements<N>(E) is expression-equivalent to elements_­view<views​::​all_­t<decltype((E))>, N>{E}.
[Example 1: auto historical_figures = map{ pair{"Lovelace"sv, 1815}, {"Turing"sv, 1912}, {"Babbage"sv, 1791}, {"Hamilton"sv, 1936} }; auto names = historical_figures | views::elements<0>; for (auto&& name : names) { cout << name << ' '; // prints Babbage Hamilton Lovelace Turing } auto birth_years = historical_figures | views::elements<1>; for (auto&& born : birth_years) { cout << born << ' '; // prints 1791 1936 1815 1912 } — end example]
keys_­view is an alias for elements_­view<R, 0>, and is useful for extracting keys from associative containers.
[Example 2: auto names = historical_figures | views::keys; for (auto&& name : names) { cout << name << ' '; // prints Babbage Hamilton Lovelace Turing } — end example]
values_­view is an alias for elements_­view<R, 1>, and is useful for extracting values from associative containers.
[Example 3: auto is_even = [](const auto x) { return x % 2 == 0; }; cout << ranges::count_if(historical_figures | views::values, is_even); // prints 2 — end example]