Defined in header <ranges> | ||
|---|---|---|
template< class R > using keys_view = ranges::elements_view<R, 0>; | (1) | (since C++20) |
namespace views {
inline constexpr auto keys = ranges::elements<0>;
}
| (2) | (since C++20) |
Takes a view of tuple-like values (e.g. std::tuple or std::pair), and produces a view with a value-type of the first element of the adapted view's value-type.
ranges::elements_view<R, 0>.views::keys(e) is expression-equivalent to keys_view<views::all_t<decltype((e))>>{e} for any suitable subexpression e.keys_view can be useful for extracting keys from associative containers, e.g.
std::map<std::string, int> map{{"one", 1}, {"two", 2}};
for (auto const& key : std::views::keys(map))
std::cout << key << ' ';
// prints: one twoDisplays values for each type of quark in particle physics.
#include <iomanip>
#include <iostream>
#include <locale>
#include <ranges>
#include <string>
#include <tuple>
#include <vector>
int main()
{
const std::vector<std::tuple<std::string, double, bool>> quark_mass_charge {
// name, MeV/c², has positive electric-charge:
{"up", 2.3, true}, {"down", 4.8, false},
{"charm", 1275, true}, {"strange", 95, false},
{"top", 173'210, true}, {"bottom", 4'180, false},
};
std::cout.imbue(std::locale("en_US.utf8"));
std::cout << "Quark name: │ ";
for (std::string const& name : std::views::keys(quark_mass_charge))
std::cout << std::setw(9) << name << " │ ";
std::cout << "\n" "Mass MeV/c²: │ ";
for (const double mass : std::views::values(quark_mass_charge))
std::cout << std::setw(9) << mass << " │ ";
std::cout << "\n" "E-charge: │ ";
for (const bool pos : std::views::elements<2>(quark_mass_charge))
std::cout << std::setw(9) << (pos ? "+2/3" : "-1/3") << " │ ";
std::cout << '\n';
}Output:
Quark name: │ up │ down │ charm │ strange │ top │ bottom │ Mass MeV/c²: │ 2.3 │ 4.8 │ 1,275 │ 95 │ 173,210 │ 4,180 │ E-charge: │ +2/3 │ -1/3 │ +2/3 │ -1/3 │ +2/3 │ -1/3 │
The following behavior-changing defect reports were applied retroactively to previously published C++ standards.
| DR | Applied to | Behavior as published | Correct behavior |
|---|---|---|---|
| LWG 3563 | C++20 | keys_view is unable to participate in CTAD due to its use of views::all_t | views::all_t removed |
|
(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++20) | takes a view consisting of tuple-like values and a number N and produces a view of N'th element of each tuple (class template) (range adaptor object) |
| BLAS-like slice of a valarray: starting index, length, stride (class) |
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