std::ranges::fold_right(3)

📦 version 2024.07.05, in nixpkgs unstable, opensuse-leap 16.0

🌸 NAME

Bestie, this is std::ranges::fold_right(3) — the C++ Standard Library algorithm that folds a range from the right side. Yes, the NAME section is literally std::ranges::fold_right - std::ranges::fold_right, because manpages love a little mirror selfie moment. 💅

✨ Synopsis

Defined in header <algorithm>.

Call signature — okay, here's the whole lineup, no crumbs:

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)

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 );
// (1)
// (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 );
// (2)
// (since C++26)

🧚 Helper concepts

template< class F, class T, class I >
concept /* indirectly-binary-left-foldable */ = /* see description */; // (3)

template< class F, class T, class I >
concept /* indirectly-binary-right-foldable */ = /* see description */; // (4)

Right-folds the elements of the given range, meaning it returns the result of evaluating 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)).

🚨 Real talk: the behavior is undefined if [first, last) is not a valid range. Not cute, not safe, not iconic. Validate your range, bestie.

The range is [first, last).
Same as (1), except that it uses r as the range, as if by using ranges::begin(r) as first and ranges::end(r) as last.
Equivalent to:

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*)

Equivalent to:

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. Slay, but make it compiler-internal. ✨

🎀 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 the 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, T>>;. Yes, that trailing semicolon is part of the vibe. Keep it.

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 = 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. No extra, no less — that's the whole budget. 💀

👑 Notes

The following table compares all constrained folding algorithms. Terminal table said "no tables," so here's the bullet-list remix:

ranges::fold_left — starts from left, initial value init, return type U.
ranges::fold_left_first — starts from left, initial value first element, return type std::optional.
ranges::fold_right — starts from right, initial value init, return type U.
ranges::fold_right_last — starts from right, initial value last element, return type std::optional.
ranges::fold_left_with_iter — starts from left, initial value init, return type:
- (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 — starts from left, initial value first element, return type:
- (1) ranges::in_value_result<I, std::optional>
- (2) ranges::in_value_result<BR, std::optional>, where BR is ranges::borrowed_iterator_t<R>

Feature-test macro

__cpp_lib_ranges_fold — value 202207L — std (C++23) — feature std::ranges fold algorithms
__cpp_lib_algorithm_default_value_type — value 202403 — std (C++26) — feature List-initialization for algorithms (1,2)

🌈 Example

Run this code, bestie — it's giving demo energy:

#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 14882:2023):
- 27.6.18 Fold [alg.fold]

🫶 See also 🫶

okay bestie, you made it to the see-also corner — these are the related pages, and yes, the whole fold crew is here 💅

ranges::fold_right_last (C++23) — right-folds a range of elements using the last element as an initial value. It's a niebloid ✨
ranges::fold_left (C++23) — left-folds a range of elements. Also a niebloid 🎀
ranges::fold_left_first (C++23) — left-folds a range of elements using the first element as an initial value, and it's a niebloid 🦋
ranges::fold_left_with_iter (C++23) — left-folds a range of elements, and returns a pair (iterator, value). Niebloid energy, periodt 💅
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. It's a function template 🌸
reduce (C++17) — similar to std::accumulate, except out of order. Also a function template 🔥

lowkey this is the whole reference list — no cap, nothing skipped 🫶

okay that's the page!! sourced from http://cppreference.com, date 2024.06.10, and the page is std::ranges::fold_right(3) — mwah, go fold something cute 💖

📦 where this page comes from

nixpkgs unstable: stdman 2024.07.05
opensuse-leap 16.0: stdman 2024.07.05-bp160.1.9 · MIT
💖 girlified by deepseek-ai/DeepSeek-V4.1-Flash-1017 on 2026-11-04: a language model's rewrite of the original, under its terms