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

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< cpp‎ | ranges‎ | 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 `base_` and the transformation function `fun_`.
2) Move constructs the underlying view `base_` from `base` and the transformation function `fun_` 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 + ...`

Run this code

    #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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