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Showing content from https://en.cppreference.com/w/cpp/language/../algorithm/../../cpp/numeric/../ranges/keys_view.html below:

std::ranges::views::keys, std::ranges::keys_view - cppreference.com

(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.

[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.

#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

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