std::ranges..._left_first(3)

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

💅 NAME

std::ranges::fold_left_first — bestie, this is literally std::ranges::fold_left_first, and it lives in the C++ Standard Library ✨

🌸 Synopsis

Miss thing is defined in header <algorithm> 🎀 and she's been serving since C++23.

✨ Call signature

Overload (1) — since C++23:

template< std::input_iterator I, std::sentinel_for<I> S,
          /*indirectly-binary-left-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_left_first( I first, S last, F f );

Overload (2) — also since C++23:

template< ranges::input_range R,
          /*indirectly-binary-left-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_left_first( R&& r, F f );

🦋 Helper concepts

Concept (3), exposition only:

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

💖 DESCRIPTION

Ok so picture this: it left-folds the elements of the given range, which is a fancy way of saying it returns the result of evaluating this chain expression, periodt:

f(f(f(f(x[1], x[2]), x[3]), ...), x[n])

where x[1], x[2], ..., x[n] are the elements of the range. 🫶

Informally — and this is the iykyk part — ranges::fold_left_first behaves like std::accumulate's overload that accepts a binary predicate, except that the *first is used internally as an initial element. Not me being the main character of accumulation 😭

⚠️ The behavior is undefined if [first, last) is not a valid range. Like, fr fr, don't do that.

1) The range is [first, last). Equivalent to return ranges::fold_left_first_with_iter(std::move(first), last, f).value.

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. Slay.

3) Equivalent to:

💀 Helper concepts

(3A) — exposition only:

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>>>;

(3B) — 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>>>>;

🧚 The niebloid lore

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. Iconic behavior honestly 💅

🍓 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 that contains the result of left-fold of the given range over f, where U is equivalent to:

decltype(ranges::fold_left(std::move(first), last, std::iter_value_t<I>(*first), f))

If the range is empty, std::optional() is returned. Yes bestie, she hands you an empty optional, no cap 😭

🔥 POSSIBLE IMPLEMENTATIONS

struct fold_left_first_fn
{
    template<std::input_iterator I, std::sentinel_for<I> S,
             /*indirectly-binary-left-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_left(std::move(first), last, std::iter_value_t<I>(*first), f)
        );
        if (first == last)
            return std::optional<U>();
        std::optional<U> init(std::in_place, *first);
        for (++first; first != last; ++first)
            *init = std::invoke(f, std::move(*init), *first);
        return std::move(init);
    }

    template<ranges::input_range R,
             /*indirectly-binary-left-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_left_first_fn fold_left_first;

⭐ COMPLEXITY

Exactly ranges::distance(first, last) - 1 (assuming the range is not empty) applications of the function object f. That's it. That's the math. 🧮✨

🌈 NOTES

The following table compares all constrained folding algorithms — I'm giving it to you as a list because we're in a terminal and also because list energy is superior 🫶

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 💖

💜 EXAMPLE

// Run this code

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

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

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

    auto mul = std::ranges::fold_left_first(v, std::multiplies<int>()); // (2)
    std::cout << "*mul: " << mul.value() << '\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 sec = std::ranges::fold_left_first
    (
        data | std::ranges::views::values, std::multiplies<>()
    );
    std::cout << "*sec: " << *sec << '\n';

    // use a program defined function object (lambda-expression):
    auto val = std::ranges::fold_left_first(v, [](int x, int y) { return x + y + 13; });
    std::cout << "*val: " << *val << '\n';
}

🦋 Output

*sum: 36
*mul: 40320
*sec: 42
*val: 127

🌸 REFERENCES

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

✨ SEE ALSO

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

ok that's the whole page bestie, you ate that 💅✨ source and version: http://cppreference.com, dated 2024.06.10, and this was std::ranges..._left_first(3) — mwah, go fold something 🫶💖

📦 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