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

NAME
     std::ranges::fold_right_last - std::ranges::fold_right_last

Synopsis
	Defined in header <algorithm>
	Call signature
	template< std::bidirectional_iterator I, std::sentinel_for<I> S,

	/*indirectly-binary-right-foldable*/<std::iter_value_t<I>, I> F >
	requires   std::constructible_from<				     (1)
     (since C++23)
		     std::iter_value_t<I>, std::iter_reference_t<I>>
	constexpr auto

	    fold_right_last( I first, S last, F f );
	template< ranges::bidirectional_range R,

		  /*indirectly-binary-right-foldable*/<
		      ranges::range_value_t<R>, ranges::iterator_t<R>> F
	>
	requires  std::constructible_from<				     (2)
     (since C++23)
		     ranges::range_value_t<R>,
	ranges::range_reference_t<R>>
	constexpr auto

	    fold_right_last( R&& r, F f );
	Helper concepts
	template<      class	  F,	  class      T,      class	I      >
     (exposition
	concept /*indirectly-binary-left-foldable*/ = /*  see  description   (3)
     only*)
	*/;
	template<      class	  F,	  class      T,      class	I      >
     (exposition
	concept /*indirectly-binary-right-foldable*/ = /*  see	description  (4)
     only*)
	*/;

	Right-folds  the elements of given range, that is, returns the result of
     evaluation
	of the chain expression:
	f(x[1], f(x[2], ...f(x[n-1], x[n]))), where x[1], x[2],  ...,  x[n]  are
     elements of
	the range.

	Informally,	    ranges::fold_right_last	    behaves	    like
     std::fold_left(ranges::reverse(r),
	*--last, /*flipped*/(f)) (assuming the range is not empty).

	The behavior is undefined if [first, last) is not a valid range.

	1)    The    range    is    [first,    last).	 Given	  U    as    de-
     cltype(ranges::fold_right(first, last,
	std::iter_value_t<I>(*first), f)), equivalent to:

      if (first == last)
	  return std::optional<U>();
      I tail = ranges::prev(ranges::next(first, std::move(last)));
      return					 std::optional<U>(std::in_place,
     ranges::fold_right(std::move(first), tail,
	  std::iter_value_t<I>(*tail), std::move(f)));

	2) Same as (1), except that  uses  r  as  the  range,  as  if  by  using
     ranges::begin(r) as
	first and ranges::end(r) as last.
	3) Equivalent to:

	Helper concepts
	template< class F, class T, class I, class U >

	concept /*indirectly-binary-left-foldable-impl*/ =
	    std::movable<T> &&
	    std::movable<U> &&
	    std::convertible_to<T, U> &&			     (3A) (expo-
     sition only*)
	    std::invocable<F&, U, std::iter_reference_t<I>> &&
	    std::assignable_from<U&,

		std::invoke_result_t<F&, U,
	std::iter_reference_t<I>>>;
	template< class F, class T, class I >

	concept /*indirectly-binary-left-foldable*/ =
	    std::copy_constructible<F> &&
	    std::indirectly_readable<I> &&
	    std::invocable<F&, T, std::iter_reference_t<I>> &&
	    std::convertible_to<std::invoke_result_t<F&, T,	     (3B) (expo-
     sition only*)
	std::iter_reference_t<I>>,
		std::decay_t<std::invoke_result_t<F&, T,
	std::iter_reference_t<I>>>> &&
	    /*indirectly-binary-left-foldable-impl*/<F, T, I,

		std::decay_t<std::invoke_result_t<F&, T,
	std::iter_reference_t<I>>>>;

	4) Equivalent to:

	Helper concepts
	template< class F, class T, class I >
									    (ex-
     position
	concept   /*indirectly-binary-right-foldable*/	 =		    (4A)
     only*)

	    /*indirectly-binary-left-foldable*/</*flipped*/<F>, T, I>;
	Helper class templates
	template< class F >

	class /*flipped*/
	{
	    F f;    // exposition only					    (ex-
     position
	public: 							    (4B)
     only*)
	    template< class T, class U >
		requires std::invocable<F&, U, T>
	    std::invoke_result_t<F&, U, T> operator()( T&&, U&& );

	};

	The  function-like  entities  described on this page are niebloids, that
     is:

	  * Explicit template argument lists cannot be	specified  when  calling
     any of them.
	  * None of them are visible to argument-dependent lookup.
	  *  When any of them are found by normal unqualified lookup as the name
     to the left
	    of the function-call operator, argument-dependent lookup  is  inhib-
     ited.

	In  practice,  they may be implemented as function objects, or with spe-
     cial compiler
	extensions.

Parameters
	first, last - the range of elements to fold
	r	    - the range of elements to fold
	f	    - the binary function object

Return value
	An object of type std::optional<U> that contains the  result  of  right-
     fold of the
	given range over f.

	If the range is empty, std::optional<U>() is returned.

Possible implementations
	struct fold_right_last_fn
	{
	    template<std::bidirectional_iterator I, std::sentinel_for<I> S,
		     /*indirectly-binary-right-foldable*/<std::iter_value_t<I>,
     I> F>
	    requires
		std::constructible_from<std::iter_value_t<I>,	std::iter_refer-
     ence_t<I>>
	    constexpr auto operator()(I first, S last, F f) const
	    {
		using U = decltype(
		    ranges::fold_right(first,				   last,
     std::iter_value_t<I>(*first), f));

		if (first == last)
		    return std::optional<U>();
		I tail = ranges::prev(ranges::next(first, std::move(last)));
		return std::optional<U>(std::in_place,
		    ranges::fold_right(std::move(first),		   tail,
     std::iter_value_t<I>(*tail),
				       std::move(f)));
	    }

	    template<ranges::bidirectional_range R,
		     /*indirectly_binary_right_foldable*/<
			 ranges::range_value_t<R>, ranges::iterator_t<R>> F>
	    requires
		std::constructible_from<ranges::range_value_t<R>,
     ranges::range_reference_t<R>>
	    constexpr auto operator()(R&& r, F f) const
	    {
		return (*this)(ranges::begin(r), ranges::end(r), std::ref(f));
	    }
	};

	inline constexpr fold_right_last_fn fold_right_last;

Complexity
	Exactly ranges::distance(first, last) applications of the  function  ob-
     ject f.

Notes
	The following table compares all constrained folding algorithms:

	     Fold function template	  Starts Initial	     Return type
					   from   value
	ranges::fold_left		  left	 init	 U
	ranges::fold_left_first 	  left	 first	 std::optional<U>
						 element
	ranges::fold_right		  right  init	 U
	ranges::fold_right_last 	  right  last	 std::optional<U>
						 element
							 (1)
     ranges::in_value_result<I, U>

	ranges::fold_left_with_iter		 left	       init	     (2)
     ranges::in_value_result<BR, U>,

							 where BR is
							 ranges::borrowed_itera-
     tor_t<R>
							 (1)
     ranges::in_value_result<I,
							 std::optional<U>>

	ranges::fold_left_first_with_iter	left	      first	     (2)
     ranges::in_value_result<BR,
						 element std::optional<U>>

							 where BR is
							 ranges::borrowed_itera-
     tor_t<R>

	 Feature-test macro    Value	Std		Feature
	__cpp_lib_ranges_fold 202207L (C++23) std::ranges fold algorithms

Example
     // Run this code

      #include <algorithm>
      #include <functional>
      #include <iostream>
      #include <ranges>
      #include <utility>
      #include <vector>

      int main()
      {
	  auto v = {1, 2, 3, 4, 5, 6, 7, 8};
	  std::vector<std::string> vs {"A", "B", "C", "D"};

	  auto	  r1	=    std::ranges::fold_right_last(v.begin(),	v.end(),
     std::plus<>()); // (1)
	  std::cout << "*r1: " << *r1 << '\n';

	  auto r2 = std::ranges::fold_right_last(vs, std::plus<>()); // (2)
	  std::cout << "*r2: " << *r2 << '\n';

	  // Use a program defined function object (lambda-expression):
	  auto r3 = std::ranges::fold_right_last(v, [](int x, int y) { return  x
     + y + 99; });
	  std::cout << "*r3: " << *r3 << '\n';

	  //  Get  the product of the std::pair::second of all pairs in the vec-
     tor:
	  std::vector<std::pair<char, float>> data  {{'A',  3.f},  {'B',  3.5f},
     {'C', 4.f}};
	  auto r4 = std::ranges::fold_right_last
	  (
	      data | std::ranges::views::values, std::multiplies<>()
	  );
	  std::cout << "*r4: " << *r4 << '\n';
      }

Output:
      *r1: 36
      *r2: ABCD
      *r3: 729
      *r4: 42

References
	  * C++23 standard (ISO/IEC 14882:2023):

	  * 27.6.18 Fold [alg.fold]

See also
	ranges::fold_right		  right-folds a range of elements
	(C++23) 			  (niebloid)
	ranges::fold_left		  left-folds a range of elements
	(C++23) 			  (niebloid)
	ranges::fold_left_first 	   left-folds  a range of elements using
     the first
	(C++23) 			  element as an initial value
					  (niebloid)
	ranges::fold_left_with_iter	  left-folds a range  of  elements,  and
     returns a pair
	(C++23) 			  (iterator, value)
					  (niebloid)
					  left-folds  a  range of elements using
     the first
	ranges::fold_left_first_with_iter element as an initial value,	and  re-
     turns a pair
	(C++23) 			  (iterator, optional)
					  (niebloid)
	accumulate			  sums up or folds a range of elements
					  (function template)
	reduce				  similar to std::accumulate, except out
     of order
	(C++17) 			  (function template)

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