std::ranges::fold_right

 

C++

Compiler support

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)

Execution support library (C++26)

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 Batch operations for_each for_each_n (C++17) Search operations all_ofany_ofnone_of (C++11)                (C++11)(C++11) countcount_if mismatch equal findfind_iffind_if_not (C++11) find_end find_first_of adjacent_find search search_n Modifying sequence operations Copy operations copycopy_if (C++11) copy_backward copy_n (C++11) move (C++11) move_backward (C++11) Swap operations swap iter_swap swap_ranges Transformation operations replacereplace_if transform replace_copyreplace_copy_if Generation operations fill fill_n generate generate_n Removing operations removeremove_if unique remove_copyremove_copy_if unique_copy Order-changing operations reverse reverse_copy rotate rotate_copy random_shuffleshuffle (until C++17)(C++11) shift_leftshift_right (C++20)(C++20) Sampling operations sample (C++17)	Sorting and related operations Partitioning operations partition partition_copy (C++11)   stable_partition is_partitioned (C++11) partition_point (C++11) Sorting operations sort stable_sort partial_sort partial_sort_copy is_sorted (C++11) is_sorted_until (C++11) nth_element Binary search operations (on partitioned ranges) lower_bound upper_bound equal_range binary_search Set operations (on sorted ranges) includes set_union set_intersection set_difference set_symmetric_difference Merge operations (on sorted ranges) merge inplace_merge Heap operations push_heap pop_heap make_heap sort_heap is_heap (C++11) is_heap_until (C++11) Minimum/maximum operations max min minmax (C++11) clamp (C++17) max_element min_element minmax_element (C++11) Lexicographical comparison operations lexicographical_compare lexicographical_compare_three_way (C++20) Permutation operations next_permutation prev_permutation is_permutation (C++11) C library qsort bsearch
Numeric operations
iota (C++11) inner_product adjacent_difference    accumulate reduce (C++17) transform_reduce (C++17)	partial_sum inclusive_scan (C++17)    exclusive_scan (C++17) transform_inclusive_scan (C++17) transform_exclusive_scan (C++17)
Operations on uninitialized memory
uninitialized_copy uninitialized_move (C++17)    uninitialized_fill uninitialized_copy_n (C++11) uninitialized_move_n (C++17) uninitialized_fill_n	destroy (C++17) destroy_n (C++17) destroy_at (C++17) construct_at (C++20)       uninitialized_default_construct (C++17) uninitialized_value_construct (C++17) uninitialized_default_construct_n (C++17) uninitialized_value_construct_n (C++17)

 

Constrained algorithms

All names in this menu belong to namespace std::ranges

Non-modifying sequence operations

all_ofany_ofnone_of for_each for_each_n    countcount_if adjacent_find mismatch equal lexicographical_compare    containscontains_subrange (C++23)(C++23) findfind_iffind_if_not find_lastfind_last_iffind_last_if_not (C++23)(C++23)(C++23)   find_end find_first_of search search_n starts_with (C++23) ends_with (C++23)

Modifying sequence operations

copycopy_if copy_n copy_backward    move move_backward shift_leftshift_right (C++23)(C++23)   fill fill_n generate generate_n    transform swap_ranges    shuffle sample removeremove_if replacereplace_if reverse rotate remove_copyremove_copy_if    unique replace_copyreplace_copy_if reverse_copy rotate_copy unique_copy

Partitioning operations

is_partitioned    partition_point    partition    partition_copy    stable_partition

Sorting operations

is_sorted is_sorted_until     sort stable_sort         partial_sort       nth_element partial_sort_copy

Binary search operations (on sorted ranges)

lower_bound         upper_bound         binary_search      equal_range

Set operations (on sorted ranges)

merge inplace_merge       set_difference set_intersection    set_union          includes set_symmetric_difference

Heap operations

is_heap is_heap_until       make_heap           sort_heap push_heap pop_heap

Minimum/maximum operations

max max_element         min min_element         minmax minmax_element     clamp

Permutation operations

is_permutation      next_permutation    prev_permutation

Fold operations

fold_left (C++23) fold_left_first (C++23)   fold_right (C++23) fold_right_last (C++23)   fold_left_with_iter (C++23) fold_left_first_with_iter (C++23)

Numeric operations iota (C++23)             Random number generation generate_random (C++26)

Operations on uninitialized storage

construct_at    destroy destroy_n destroy_at uninitialized_copy    uninitialized_move uninitialized_fill uninitialized_copy_n    uninitialized_move_n uninitialized_fill_n uninitialized_default_construct uninitialized_default_construct_n uninitialized_value_construct uninitialized_value_construct_n

Return types

in_fun_result in_in_result in_found_result    in_out_result in_in_out_result    in_out_out_result    min_max_result out_value_result (C++23) in_value_result (C++23)

 

Defined in header <algorithm>
Call signature
	(1)
template< std::bidirectional_iterator I, std::sentinel_for<I> S, class T,           /* indirectly-binary-right-foldable */<T, I> F > constexpr auto fold_right( I first, S last, T init, F f );			(since C++23) (until C++26)
template< std::bidirectional_iterator I, std::sentinel_for<I> S,           class T = std::iter_value_t<I>,           /* indirectly-binary-right-foldable */<T, I> F > constexpr auto fold_right( I first, S last, T init, F f );			(since C++26)
		(2)
template< ranges::bidirectional_range R, class T,           /* indirectly-binary-right-foldable */               <T, ranges::iterator_t<R>> F > constexpr auto fold_right( R&& r, T init, F f );				(since C++23) (until C++26)
template< ranges::bidirectional_range R, class T = ranges::range_value_t<R>,           /* indirectly-binary-right-foldable */               <T, ranges::iterator_t<R>> F > constexpr auto fold_right( R&& r, T init, F f );				(since C++26)
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, init))), where x1, x2, ..., xn are elements of the range.

Informally, ranges::fold_right behaves like std::fold_left(ranges::reverse(r), init, /* flipped */(f)).

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

1) The range is [first, last).

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
init	-	the initial value of the fold
f	-	the binary function object

Return value

An object of type U that contains the result of right-fold of the given range over f, where U is equivalent to std::decay_t<std::invoke_result_t<F&, std::iter_reference_t<I>, T>>;.

If the range is empty, U(std::move(init)) is returned.

Possible implementations

struct fold_right_fn { template<std::bidirectional_iterator I, std::sentinel_for<I> S, class T = std::iter_value_t<I>, /* indirectly-binary-right-foldable */<T, I> F> constexpr auto operator()(I first, S last, T init, F f) const { using U = std::decay_t<std::invoke_result_t<F&, std::iter_reference_t<I>, T>>; if (first == last) return U(std::move(init)); I tail = ranges::next(first, last); U accum = std::invoke(f, *--tail, std::move(init)); while (first != tail) accum = std::invoke(f, *--tail, std::move(accum)); return accum; }   template<ranges::bidirectional_range R, class T = ranges::range_value_t<R>, /* indirectly-binary-right-foldable */<T, ranges::iterator_t<R>> F> constexpr auto operator()(R&& r, T init, F f) const { return (*this)(ranges::begin(r), ranges::end(r), std::move(init), std::ref(f)); } };   inline constexpr fold_right_fn fold_right;

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) 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
__cpp_lib_algorithm_default_value_type	202403L	(C++26)	List-initialization for algorithms (1,2)

Example

Run this code

#include <algorithm>
#include <complex>
#include <functional>
#include <iostream>
#include <ranges>
#include <string>
#include <utility>
#include <vector>
 
using namespace std::literals;
namespace ranges = std::ranges;
 
int main()
{
    auto v = {1, 2, 3, 4, 5, 6, 7, 8};
    std::vector<std::string> vs{"A", "B", "C", "D"};
 
    auto r1 = ranges::fold_right(v.begin(), v.end(), 6, std::plus<>()); // (1)
    std::cout << "r1: " << r1 << '\n';
 
    auto r2 = ranges::fold_right(vs, "!"s, std::plus<>()); // (2)
    std::cout << "r2: " << r2 << '\n';
 
    // Use a program defined function object (lambda-expression):
    std::string r3 = ranges::fold_right
    (
        v, "A", [](int x, std::string s) { return s + ':' + std::to_string(x); }
    );
    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', 2.f}, {'B', 3.f}, {'C', 3.5f}};
    float r4 = ranges::fold_right
    (
        data | ranges::views::values, 2.0f, std::multiplies<>()
    );
    std::cout << "r4: " << r4 << '\n';
 
    using CD = std::complex<double>;
    std::vector<CD> nums{{1, 1}, {2, 0}, {3, 0}};
    #ifdef __cpp_lib_algorithm_default_value_type
        auto r5 = ranges::fold_right(nums, {7, 0}, std::multiplies{});
    #else
        auto r5 = ranges::fold_right(nums, CD{7, 0}, std::multiplies{});
    #endif
    std::cout << "r5: " << r5 << '\n';
}

Output:

r1: 42
r2: ABCD!
r3: A:8:7:6:5:4:3:2:1
r4: 42
r5: (42,42)

References

• C++23 standard (ISO/IEC 14882:2024):

• 27.6.18 Fold [alg.fold]

See also

ranges::fold_right_last (C++23)	right-folds a range of elements using the last element as an initial value (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)