std::ranges:: views:: keys, std::ranges:: keys_view
|
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.
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
Example
Displays 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 │
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 |
See also
takes a
view
consisting of pair-like values and produces a
view
of the second elements of each pair
(class template) (range adaptor object) |
|
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) |