std::ranges::views::keys, std::ranges::keys_view
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C++
Feature test macros (C++20)
Concepts library (C++20)
Metaprogramming library (C++11)
Ranges library (C++20)
Filesystem library (C++17)
Concurrency support library (C++11)
Execution control library (C++26)
Ranges library
takes a
(class template) (range adaptor object)[edit]
(C++23)(C++23)
(C++23)(C++23)
(C++23)(C++23)
(C++26)(C++26)
(C++23)(C++23)
keys_viewviews::keys
(C++23)(C++23)
(C++23)(C++23)
(C++23)(C++23)
(C++23)(C++23)
(C++23)
(C++23)(C++23)
(C++23)
(C++23)(C++23)
(C++23)(C++23)
(C++23)(C++23)
(C++23)(C++23)
(C++23)(C++23)
(C++23)
Defined in header
<ranges>
template< class R >
using keys_view = ranges::elements_view <R, 0>;
(1)
(since C++20)
using keys_view = ranges::elements_view <R, 0>;
namespace views {
(2)
(since C++20)
inline constexpr auto keys = ranges::elements<0>;
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.
1) An alias for ranges::elements_view <R, 0>.
2) RangeAdaptorObject (and also RangeAdaptorClosureObject). The expression views::keys(e) is expression-equivalent to keys_view<views::all_t <decltype((e))>>{e} for any suitable subexpression e.
Contents
[edit] Notes
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 two
[edit] Example
Displays values for each type of quark in particle physics.
Run this code
#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 │
[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 3563 | C++20 | keys_view is unable to participate in CTAD due to its use of views::all_t
|
views::all_t removed |
[edit] See also
(C++20)
view consisting of pair-like values and produces a view of the second elements of each pair(class template) (range adaptor object)[edit]
takes a
(class template) (range adaptor object)[edit]
view consisting of tuple-like values and a number N and produces a view of Nth element of each tuple(class template) (range adaptor object)[edit]