std::ranges::fold_right_last

 

C++

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Freestanding and hosted

Language

Standard library

Standard library headers

Named requirements

Feature test macros (C++20)

Language support library

Concepts library (C++20)

Metaprogramming library (C++11)

Diagnostics library

General utilities library

Strings library

Containers library

Iterators library

Ranges library (C++20)

Algorithms library

Numerics library

Localizations library

Input/output library

Filesystem library (C++17)

Regular expressions library (C++11)

Concurrency support library (C++11)

Technical specifications

Symbols index

External libraries

 

Algorithm library

Constrained algorithms and algorithms on ranges (C++20)
Constrained algorithms, e.g. ranges::copy, ranges::sort, ...
Execution policies (C++17)
is_execution_policy (C++17)	execution::seqexecution::parexecution::par_unseqexecution::unseq (C++17)(C++17)(C++17)(C++20)	execution::sequenced_policyexecution::parallel_policyexecution::parallel_unsequenced_policyexecution::parallel_unsequenced (C++17)(C++17)(C++17)(C++20)
Non-modifying sequence operations
all_ofany_ofnone_of (C++11)(C++11)(C++11) for_each for_each_n (C++17)	countcount_if mismatch equal adjacent_find	search search_n lexicographical_compare lexicographical_compare_three_way (C++20)	findfind_iffind_if_not (C++11) find_end find_first_of
Modifying sequence operations
copycopy_if (C++11) copy_n (C++11) copy_backward move (C++11) move_backward (C++11) shift_leftshift_right (C++20)(C++20) transform	fill fill_n generate generate_n swap iter_swap swap_ranges sample (C++17)	removeremove_if replacereplace_if reverse rotate unique random_shuffle (until C++17)	remove_copyremove_copy_if replace_copyreplace_copy_if reverse_copy rotate_copy unique_copy shuffle (C++11)
Partitioning operations
is_partitioned (C++11) partition_point (C++11)	partition partition_copy (C++11)	stable_partition
Sorting operations
is_sorted (C++11) is_sorted_until (C++11)	sort stable_sort	partial_sort partial_sort_copy	nth_element
Binary search operations
lower_bound	upper_bound	binary_search	equal_range
Set operations (on sorted ranges)
merge inplace_merge	set_difference set_intersection	set_symmetric_difference set_union	includes
Heap operations
is_heap (C++11) is_heap_until (C++11)	make_heap sort_heap	push_heap pop_heap
Minimum/maximum operations
max max_element	min min_element	minmax (C++11) minmax_element (C++11)	clamp (C++17)
Permutations
is_permutation (C++11)	next_permutation	prev_permutation
Numeric operations
iota (C++11) inner_product adjacent_difference	accumulate reduce (C++17) transform_reduce (C++17)	transform_inclusive_scan (C++17) transform_exclusive_scan (C++17)	partial_sum inclusive_scan (C++17) exclusive_scan (C++17)
Operations on uninitialized storage
uninitialized_copy uninitialized_move (C++17) uninitialized_fill	uninitialized_copy_n (C++11) uninitialized_move_n (C++17) uninitialized_fill_n	uninitialized_default_construct (C++17) uninitialized_value_construct (C++17) uninitialized_default_construct_n (C++17) uninitialized_value_construct_n (C++17)	destroy (C++17) destroy_n (C++17) destroy_at (C++17) construct_at (C++20)
C library
qsort	bsearch

 

Constrained algorithms

Non-modifying sequence operations
ranges::all_ofranges::any_ofranges::none_of ranges::for_each ranges::for_each_n ranges::lexicographical_compare      ranges::adjacent_find	ranges::countranges::count_if ranges::mismatch ranges::equal ranges::containsranges::contains_subrange (C++23)(C++23)	ranges::findranges::find_ifranges::find_if_not ranges::find_lastranges::find_last_ifranges::find_last_if_not (C++23)(C++23)(C++23)	ranges::find_end ranges::find_first_of ranges::search ranges::search_n ranges::starts_with (C++23) ranges::ends_with (C++23)
Modifying sequence operations
ranges::copyranges::copy_if ranges::copy_n ranges::copy_backward ranges::move ranges::move_backward ranges::shift_leftranges::shift_right (C++23)(C++23)   ranges::transform	ranges::fill ranges::fill_n ranges::generate ranges::generate_n ranges::swap_ranges ranges::shuffle ranges::sample	ranges::removeranges::remove_if ranges::replaceranges::replace_if ranges::reverse ranges::rotate ranges::unique	ranges::remove_copyranges::remove_copy_if ranges::replace_copyranges::replace_copy_if ranges::reverse_copy ranges::rotate_copy ranges::unique_copy
Partitioning operations
ranges::is_partitioned ranges::partition_point	ranges::partition ranges::partition_copy	ranges::stable_partition
Sorting operations
ranges::is_sorted ranges::is_sorted_until	ranges::sort ranges::stable_sort	ranges::partial_sort ranges::partial_sort_copy	ranges::nth_element
Binary search operations (on sorted ranges)
ranges::lower_bound	ranges::upper_bound	ranges::binary_search	ranges::equal_range
Set operations (on sorted ranges)
ranges::merge ranges::inplace_merge	ranges::set_difference ranges::set_intersection	ranges::set_symmetric_difference    ranges::set_union	ranges::includes
Heap operations
ranges::is_heap ranges::is_heap_until	ranges::make_heap ranges::sort_heap	ranges::push_heap ranges::pop_heap
Minimum/maximum operations
ranges::max ranges::max_element	ranges::min ranges::min_element	ranges::minmax ranges::minmax_element	ranges::clamp
Permutation operations
ranges::is_permutation	ranges::next_permutation	ranges::prev_permutation
Numeric operations
ranges::iota (C++23)
Fold operations
ranges::fold_left (C++23) ranges::fold_left_first (C++23)	ranges::fold_right (C++23) ranges::fold_right_last (C++23)	ranges::fold_left_with_iter (C++23) ranges::fold_left_first_with_iter (C++23)
Operations on uninitialized storage
ranges::construct_at ranges::destroy ranges::destroy_n ranges::destroy_at	ranges::uninitialized_copy    ranges::uninitialized_move ranges::uninitialized_fill	ranges::uninitialized_copy_n    ranges::uninitialized_move_n ranges::uninitialized_fill_n	ranges::uninitialized_default_construct ranges::uninitialized_default_construct_n    ranges::uninitialized_value_construct ranges::uninitialized_value_construct_n
Return types
ranges::in_fun_result ranges::in_in_result ranges::in_found_result	ranges::in_out_result ranges::in_in_out_result	ranges::in_out_out_result ranges::min_max_result	ranges::out_value_result (C++23) ranges::in_value_result (C++23)

 

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<              std::iter_value_t<I>, std::iter_reference_t<I>> constexpr auto fold_right_last( I first, S last, F f );	(1)	(since C++23)
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 fold_right_last( R&& r, F f );	(2)	(since C++23)
Helper concepts
template< class F, class T, class I > concept __indirectly_binary_left_foldable = /* see description */;	(3)	(exposition only*)
template< class F, class T, class I > concept __indirectly_binary_right_foldable = /* see description */;	(4)	(exposition only*)

Right-folds the elements of given range, that is, returns the result of evaluation of the chain expression:
f(x1, f(x2, ...f(xn-1, xn))), where x1, x2, ..., xn 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> &&   std::invocable<F&, U, std::iter_reference_t<I>> &&   std::assignable_from<U&,     std::invoke_result_t<F&, U, std::iter_reference_t<I>>>;	(3A)	(exposition only*)
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, 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>>>>;	(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 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 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) std::in_value_result<I, U> (2) std::in_value_result<BR, U>, where BR is ranges::borrowed_iterator_t<R>
ranges::fold_left_first_with_iter	left	first element	(1) std::in_value_result<I, std::optional<U>> (2) std::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

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