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std::ranges::transform_view<V,F>::transform_view

transform_view() requires std::default_initializable<V> &&
                          std::default_initializable<F> = default;
 (1) (since C++20) 
constexpr explicit transform_view( V base, F fun );
 (2) (since C++20)

Constructs a transform_view.

1) Default constructor. Value-initializes the underlying view and the transformation function.
2) Move constructs the underlying view from base and the transformation function from fun.

Parameters

base - view
fun - transformation function

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 + ...

#include <iomanip>
#include <iostream>
#include <numbers>
#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) | atan1term;
        const double accum = std::accumulate(seq.begin(), seq.end(), 0.0);
        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)

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 constructor
constructs a transform_view which does not contain an F		 the transform_view is also
not default_initializable

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