std::ranges::transform_view<V,F>::transform_view
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std::ranges::transform_view
transform_view::transform_view
(C++26)
transform_view() requires std::default_initializable <V> &&
std::default_initializable <F> = default;
(1)
(since C++20)
std::default_initializable <F> = default;
constexpr explicit transform_view( V base, F fun );
(2)
(since C++20)
Constructs a transform_view.
1) Default constructor. Value-initializes the underlying view
base_ and the transformation function fun_ .2) Move constructs the underlying view
base_ from base and the transformation function fun_ from fun.[edit] Parameters
base
-
view
fun
-
transformation function
[edit] Example
Demonstrates π approximation using serial expansion of arc tangent of 1: atan(1) = π/4 ≈ 1 - 1/3 + 1/5 - 1/7 + 1/9 - 1/11 + ...
Run this code
#include <algorithm> #include <functional> #include <iomanip> #include <iostream> #include <numeric> #include <ranges> int main() { std::cout << std::setprecision (15) << std::fixed ; auto atan1term{std::ranges::views::transform ( [](int n) { return ((n % 2) ? -1 : 1) * 1.0 / (2 * n + 1); } )}; for (const int iterations : {1, 2, 3, 4, 5, 10, 100, 1000, 1'000'000}) { auto seq{std::ranges::views::iota (0, iterations) bitor atan1term}; const double accum{*std::ranges::fold_left_first (seq, std::plus {})}; std::cout << "π ≈ " << 4 * accum << " (iterations: " << iterations << ")\n"; } std::cout << "π ≈ " << std::numbers::pi << " (std::numbers::pi)\n"; }
Possible output:
π ≈ 4.000000000000000 (iterations: 1) π ≈ 2.666666666666667 (iterations: 2) π ≈ 3.466666666666667 (iterations: 3) π ≈ 2.895238095238096 (iterations: 4) π ≈ 3.339682539682540 (iterations: 5) π ≈ 3.041839618929403 (iterations: 10) π ≈ 3.131592903558554 (iterations: 100) π ≈ 3.140592653839794 (iterations: 1000) π ≈ 3.141591653589774 (iterations: 1000000) π ≈ 3.141592653589793 (std::numbers::pi)
[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 |
| P2325R3 | C++20 | if F is not default_initializable, the default constructorconstructs a transform_view which does not contain an F
|
the transform_view is alsonot default_initializable
|