std::ranges::fold_left(3)

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

std::ranges::fold_left(3) 💅

Ok so this is the C++ Standard Libary manual page for std::ranges::fold_left, and we are doing the whole thing: signatures, niebloids, feature-test macros, the example, all of it. Buckle up bestie 🎀

✨ NAME ✨

std::ranges::fold_left - std::ranges::fold_left

🌸 Synopsis 🌸

Defined in header <algorithm>

Call signature

template< std::input_iterator I, std::sentinel_for<I> S, class T,
          /* indirectly-binary-left-foldable */<T, I> F >
constexpr auto fold_left( I first, S last, T init, F f );
// (since C++23) (until C++26)

template< std::input_iterator I, std::sentinel_for<I> S,
          class T = std::iter_value_t<I>,
          /* indirectly-binary-left-foldable */<T, I> F >
constexpr auto fold_left( I first, S last, T init, F f ); // (1) (since C++26)

template< ranges::input_range R, class T,
          /* indirectly-binary-left-foldable */<T, ranges::iterator_t<R>> F >
constexpr auto fold_left( R&& r, T init, F f );
// (since C++23) (until C++26)

template< ranges::input_range R, class T = ranges::range_value_t<R>,
          /* indirectly-binary-left-foldable */<T, ranges::iterator_t<R>> F >
constexpr auto fold_left( 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) (exposition only*)

Left-folds the elements of given range, that is, returns the result of evaluation of the chain expression: f(f(f(f(init, x[1]), x[2]), ...), x[n]), where x[1], x[2], ..., x[n] are elements of the range.

Informally, ranges::fold_left behaves like std::accumulate's overload that accepts a binary predicate.

The behavior is undefined if [first, last) is not a valid range. Not me being dramatic, that's just the rules 😭

The range is [first, last). Equivalent to return ranges::fold_left_with_iter(std::move(first), last, std::move(init), f).value.
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.
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> &&                             // (3A) (exposition only*)
    std::invocable<F&, U, std::iter_reference_t<I>> &&
    std::assignable_from<U&,
        std::invoke_result_t<F&, U, std::iter_reference_t<I>>>;

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,          // (3B) (exposition only*)
        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>>>>;

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 left-fold of the given range over f, where U is equivalent to std::decay_t<std::invoke_result_t<F&, T, std::iter_reference_t>>.

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

🦋 Possible implementations 🦋

struct fold_left_fn {
    template<std::input_iterator I, std::sentinel_for<I> S, class T = std::iter_value_t<I>,
             /* indirectly-binary-left-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&, T, std::iter_reference_t<I>>>;
        if (first == last)
            return U(std::move(init));
        U accum = std::invoke(f, std::move(init), *first);
        for (++first; first != last; ++first)
            accum = std::invoke(f, std::move(accum), *first);
        return std::move(accum);
    }

    template<ranges::input_range R, class T = ranges::range_value_t<R>,
             /* indirectly-binary-left-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_left_fn fold_left;

⭐ Complexity ⭐

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

🌈 Notes 🌈

The following table compares all constrained folding algorithms:

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

#include <algorithm>
#include <complex>
#include <functional>
#include <iostream>
#include <ranges>
#include <string>
#include <utility>
#include <vector>

int main()
{
    namespace ranges = std::ranges;

    std::vector v{1, 2, 3, 4, 5, 6, 7, 8};

    int sum = ranges::fold_left(v.begin(), v.end(), 0, std::plus<int>()); // (1)
    std::cout << "sum: " << sum << '\n';

    int mul = ranges::fold_left(v, 1, std::multiplies<int>()); // (2)
    std::cout << "mul: " << mul << '\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 sec = ranges::fold_left
    (
        data | ranges::views::values, 2.0f, std::multiplies<>()
    );
    std::cout << "sec: " << sec << '\n';

    // use a program defined function object (lambda-expression):
    std::string str = ranges::fold_left
    (
        v, "A", [](std::string s, int x) { return s + ':' + std::to_string(x); }
    );
    std::cout << "str: " << str << '\n';

    using CD = std::complex<double>;
    std::vector<CD> nums{{1, 1}, {2, 0}, {3, 0}};
    #ifdef __cpp_lib_algorithm_default_value_type
        auto res = ranges::fold_left(nums, {7, 0}, std::multiplies{}); // (2)
    #else
        auto res = ranges::fold_left(nums, CD{7, 0}, std::multiplies{}); // (2)
    #endif
    std::cout << "res: " << res << '\n';
}

💜 Output 💜

sum: 36
mul: 40320
sec: 42
str: A:1:2:3:4:5:6:7:8
res: (42,42)

🔥 References 🔥

C++23 standard (ISO/IEC 14882:2023):
27.6.18 Fold [alg.fold]

👑 See also 👑

ranges::fold_left_first (C++23) — left-folds a range of elements using the first element as an initial value (niebloid)
ranges::fold_right (C++23) — right-folds a range of elements (niebloid)
ranges::fold_right_last (C++23) — right-folds a range of elements using the last 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)

Source/version/date vibe check: http://cppreference.com · 2024.06.10 · page std::ranges::fold_left(3) — mwah 💋

📦 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