std::ranges::fold_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 decltype(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) (exposition 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) (exposition 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 >
                        (exposition
   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 (exposition
   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 inhibited.
   In practice, they may be implemented as function objects, or with special 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_reference_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 object 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_iterator_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_iterator_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 vector:
     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 returns 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)