std::ranges...w::take_view(3)

✨ NAME ✨

okay so bestie, this is the std::ranges...w::take_view(3) page from the C++ Standard Libary manual, and the main character is literally std::ranges::take_view::take_view — std::ranges::take_view::take_view. yes, that's the whole NAME. constructor energy only. 💅

🌸 Synopsis 🌸

take_view() requires std::default_initializable<V> = default;
(1) (since C++20)
constexpr explicit take_view( V base,
(2) (since C++20)
ranges::range_difference_t<V> count );

💖 Parameters 💖

• base — the underlying view 🫶
• count — number of elements to take 🔢

🦋 Example 🦋

Prints first n prime numbers that are generated using Sieve of Eratosthenes method. (yes, that is the whole vibe, no cap.)

        #include <bit>
        #include <bitset>
        #include <iomanip>
        #include <iostream>
        #include <limits>
        #include <ranges>

        constexpr unsigned clog2(auto x) // ~ ⌈ log₂(x) ⌉
        {
            return std::numeric_limits<decltype(x)>::digits -
       std::countl_zero(x);
        }

        template<unsigned Count>
        struct FirstPrimes
        {
            static constexpr int count = Count;

            constexpr bool operator()(int n) // is prime?
            {
                return n < 2 ? false :
                       n == 2 ? true :
                       n % 2 == 0 or bits_.test(n / 2) ? false : true;
            }
        private:
            consteval static auto init()
            {
                std::bitset<size_ / 2 + 1> bits;
                for (int n{3}; n < size_; n += 2)
                    for (int i{n}, j{3}, k{}; (k = i * j) < size_; j += 2)
                        bits.set(k / 2);
                return bits;
            }

            // Keep only odd numbers; 0 means it is a prime
            constexpr static auto bits_ { init() };

            // a(n) <= n * (log(n) + log(log(n)))
            static constexpr int size_ = Count * (clog2(Count) +
       clog2(clog2(Count)));
        };

        int main()
        {
            constexpr FirstPrimes<42> primes;

            auto primes_view = std::ranges::take_view{ std::views::iota(1)
                                                     |
       std::views::filter(primes)
                                                     , primes.count };

            std::cout << "First " << primes.count << " prime numbers are:\n";
            for (int new_line{1}; const int prime : primes_view)
                std::cout << std::setw(3) << prime << (new_line++ % 7 ? ' ' :
       '\n');
        }

Output 🍓

        First 42 prime numbers are:
          2   3   5   7  11  13  17
         19  23  29  31  37  41  43
         47  53  59  61  67  71  73
         79  83  89  97 101 103 107
        109 113 127 131 137 139 149
        151 157 163 167 173 179 181

🔥 Defect reports 🔥

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

• DR: LWG 3714 (P2711R1) — Applied to: C++20 — Behavior as published: the multi-parameter constructor was not made explicit — Correct behavior: explicit.

mwah 💋 this iconic page is brought to you by http://cppreference.com, version/date 2024.06.10, and it's the std::ranges...w::take_view(3) page. byeee 🫶🎀