std::ranges::fold_right_last

Right-folds the elements of given range, that is, returns the result of evaluation of the chain expression:
f(x₁, f(x₂, ...f(x_n-1, x_n))), where x₁, x₂, ..., x_n are elements of the range.

Informally, ranges::fold_right_last behaves like ranges::fold_left (views::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 (*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 &&
std::convertible_to <T, U> &&
std::invocable <F&, U, std::iter_reference_t > &&
std::assignable_from <U&,

std::invoke_result_t <F&, U, std::iter_reference_t >>;

(3A) (exposition only*)

template< class F, class T, class I >

concept /*indirectly-binary-left-foldable*/ =
std::copy_constructible <F> &&
std::indirectly_readable &&
std::invocable <F&, T, std::iter_reference_t > &&
std::convertible_to <std::invoke_result_t <F&, T, std::iter_reference_t >,
std::decay_t <std::invoke_result_t <F&, T, std::iter_reference_t >>> &&
/*indirectly-binary-left-foldable-impl*/<F, T, I,

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

(3B) (exposition only*)

4) Equivalent to:

Helper concepts

template< class F, class T, class I >

concept /*indirectly-binary-right-foldable*/ =

/*indirectly-binary-left-foldable*/</*flipped*/<F>, T, I>;

(4A) (exposition only*)

Helper class templates

template< class F >

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

};

(4B) (exposition only*)

The function-like entities described on this page are algorithm function objects (informally known as 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.

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;

[edit] Complexity

Exactly ranges::distance (first, last) applications of the function object f.

[edit] Notes

The following table compares all constrained folding algorithms:

Fold function template	Starts from	Initial value	Return type
ranges::fold_left	left	init	U
ranges::fold_left_first	left	first element	std::optional <U>
ranges::fold_right	right	init	U
ranges::fold_right_last	right	last element	std::optional <U>
ranges::fold_left_with_iter	left	init	(1) ranges::in_value_result <I, U> (2) ranges::in_value_result <BR, U>, where BR is ranges::borrowed_iterator_t <R>
ranges::fold_left_first_with_iter	left	first element	(1) ranges::in_value_result <I, std::optional <U>> (2) ranges::in_value_result <BR, 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

[edit] 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';
}