std::ranges::take_view<V>::take_view
|
(1) | (since C++20) |
|
(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.
#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 181Defect 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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