/C++

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

 ```transform_view() requires std::default_initializable && std::default_initializable = default;``` (1) (since C++20) `constexpr 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 <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}) {
double accum{0.0};
for (double term : std::ranges::views::iota(0, iterations) | atan1term) {
accum += term;
}
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
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`