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std::ranges::take_view<V>::take_view

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

Constructs a take_view.

1) Default constructor. Value-initializes the underlying view and initializes the count to 0. After construction, base() returns a copy of V() and size() returns 0.

2) Initializes the underlying view with std::move(base) and the count with count. After construction, base() returns a copy of base and size() returns the smaller of count and ranges::size(base).

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.

// https://godbolt.org/z/T76GG9dr1
#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	Applied to	Behavior as published	Correct behavior
LWG 3714 (P2711R1)	C++20	the multi-parameter constructor was not explicit	made explicit

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