NAME¶

std::ranges::take_view::take_view - std::ranges::take_view::take_view

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 );

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.

// Run this code

#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 C++20 the multi-parameter constructor was not made explicit
(P2711R1) explicit