The following code example is taken from the book
C++20 - The Complete Guide by Nicolai M. Josuttis, Leanpub, 2021
The code is licensed under a Creative Commons Attribution 4.0 International License. Creative Commons License

// raw code

#include <iostream>
#include <string>
#include <unordered_map>
#include <ranges>
#include <numbers>    // for math constants
#include <algorithm>  // for sort()

void print(std::ranges::input_range auto&& coll)
  for (const auto& elem : coll) {
    std::cout << elem << ' ';
  std::cout << '\n';

int main()
  std::unordered_map<std::string, double> coll{
      {"pi", std::numbers::pi},
      {"e", std::numbers::e},
      {"golden-ratio", std::numbers::egamma},
      {"euler-constant", std::numbers::phi},


  print(coll | std::views::keys);
  print(coll | std::views::values);