std::ranges:: fold_right

From cppreference.com

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(x ₁ , f(x ₂ , ...f(x _n , init))) , where x ₁ , x ₂ , ..., x _n 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 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.

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]

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

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