std::ranges::take_view<V>::take_view
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Ranges library
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(C++23)
std::ranges::take_view
take_view::take_view
(C++26)
Member functions
Non-member functions
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
base_ and initializes the count_ to 0. After construction, base() returns a copy of V() and size() returns 0.2) Initializes the underlying view
base_ 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).[edit] Parameters
base
-
the underlying view
count
-
number of elements to take
[edit] 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
[edit] 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 |