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std::ranges...ansform_view(3)  C++ Standard Libary std::ranges...ansform_view(3)

NAME
     std::ranges::transform_view::transform_view      -      std::ranges::trans-
     form_view::transform_view

Synopsis
	transform_view() requires std::default_initializable<V>  &&	     (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	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 I approximation using serial expansion of arc tangent of 1:
	atan(1) = I/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 << "I ~ " << 4 * accum << " (iterations: " << iterations
     << ")\n";
	  }
	  std::cout << "I ~ " << std::numbers::pi << " (std::numbers::pi)\n";
      }

Possible output:
      I ~ 4.000000000000000 (iterations: 1)
      I ~ 2.666666666666667 (iterations: 2)
      I ~ 3.466666666666667 (iterations: 3)
      I ~ 2.895238095238096 (iterations: 4)
      I ~ 3.339682539682540 (iterations: 5)
      I ~ 3.041839618929403 (iterations: 10)
      I ~ 3.131592903558554 (iterations: 100)
      I ~ 3.140592653839794 (iterations: 1000)
      I ~ 3.141591653589774 (iterations: 1000000)
      I ~ 3.141592653589793 (std::numbers::pi)

	Defect reports

	The following behavior-changing defect	reports  were  applied	retroac-
     tively to
	previously published C++ standards.

	   DR	   Applied  to	      Behavior as published		 Correct
     behavior
	LWG 3714  C++20      the multi-parameter constructor was  made explicit
	(P2711R1)	     not explicit
			     if F is not default_initializable,
	P2325R3   C++20      the  default  constructor		     the  trans-
     form_view is also
			     constructs   a   transform_view  which	not  de-
     fault_initializable
			     does not contain an F

http://cppreference.com 	   2024.06.10	   std::ranges...ansform_view(3)
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