std::ranges..._right_last(3)

💖 NAME 💖

okay so bestie, welcome to the std::ranges::fold_right_last(3) page from the C++ Standard Library manual 🫶. Yes the title is a mouthful, but it's literally right-fold-but-last-element-core, and we're gonna get through it together.

std::ranges::fold_right_last - std::ranges::fold_right_last

✨ Synopsis ✨

Defined in header <algorithm> — iconic, obviously.

Call signature

Signature (1), since C++23:

template< std::bidirectional_iterator I, std::sentinel_for<I> S,
          /*indirectly-binary-right-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_right_last( I first, S last, F f );

Signature (2), since C++23:

template< ranges::bidirectional_range R,
          /*indirectly-binary-right-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_right_last( R&& r, F f );

Helper concepts

Left-foldable helper concept, exposition only, (3):

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

Right-foldable helper concept, exposition only, (4):

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

Right-folds the elements of given range — meaning it returns the result of evaluating this chain expression:

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

where x[1], x[2], ..., x[n] are elements of the range. Yes queen, right fold. 💅

Informally, ranges::fold_right_last behaves like std::fold_left(ranges::reverse(r), *--last, /*flipped*/(f)) (assuming the range is not empty).

⚠️ The behavior is undefined if [first, last) is not a valid range. Not a vibe. Keep your range valid, bestie.

1) The range is [first, last). Given U as decltype(ranges::fold_right(first, last, std::iter_value_t<I>(*first), f)), equivalent to:

if (first == last)
    return std::optional<U>();
I tail = ranges::prev(ranges::next(first, std::move(last)));
return std::optional<U>(std::in_place,
ranges::fold_right(std::move(first), tail,
std::iter_value_t<I>(*tail), std::move(f)));

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>>>;
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>>>>;

(3A) and (3B) are both 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) is 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) is 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.
• f — the binary function object.

🌸 Return value 🌸

An object of type std::optional<U> that contains the result of right-fold of the given range over f.

If the range is empty, std::optional<U>() is returned. Cute empty-range behavior, no cap.

💅 Possible implementations 💅

struct fold_right_last_fn
{
    template<std::bidirectional_iterator I, std::sentinel_for<I> S,
             /*indirectly-binary-right-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_right(first, last, std::iter_value_t<I>(*first), f));

        if (first == last)
            return std::optional<U>();
        I tail = ranges::prev(ranges::next(first, std::move(last)));
        return std::optional<U>(std::in_place,
            ranges::fold_right(std::move(first), tail,
                               std::iter_value_t<I>(*tail),
                               std::move(f)));
    }

    template<ranges::bidirectional_range R,
             /*indirectly_binary_right_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_right_last_fn fold_right_last;

🔥 Complexity 🔥

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

🎀 Notes 🎀

The following table compares all constrained folding algorithms — I'm giving it to you as a list because tables are not the vibe here:

• 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<U>.
• 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<U>.
• 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<U>>; (2) ranges::in_value_result<BR, std::optional<U>>, where BR is ranges::borrowed_iterator_t<R>.

Feature-test macro:

• Feature-test macro — __cpp_lib_ranges_fold — value 202207L — std (C++23) — feature std::ranges fold algorithms.

🌈 Example 🌈

Run this code, bestie:

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

int main()
{
    auto v = {1, 2, 3, 4, 5, 6, 7, 8};
    std::vector<std::string> vs {"A", "B", "C", "D"};

    auto r1 = std::ranges::fold_right_last(v.begin(), v.end(),
std::plus<>()); // (1)
    std::cout << "*r1: " << *r1 << '\n';

    auto r2 = std::ranges::fold_right_last(vs, std::plus<>()); // (2)
    std::cout << "*r2: " << *r2 << '\n';

    // Use a program defined function object (lambda-expression):
    auto r3 = std::ranges::fold_right_last(v, [](int x, int y) { return
x + y + 99; });
    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', 3.f}, {'B', 3.5f},
{'C', 4.f}};
    auto r4 = std::ranges::fold_right_last
    (
        data | std::ranges::views::values, std::multiplies<>()
    );
    std::cout << "*r4: " << *r4 << '\n';
}

Output

*r1: 36
*r2: ABCD
*r3: 729
*r4: 42

⭐ References ⭐

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

👑 See also 👑

• ranges::fold_right (C++23) — right-folds a range of elements (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)

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okay that's the whole page, bestie. Source: http://cppreference.com · version/date: 2024.06.10 · page: std::ranges..._right_last(3) — now go fold right and slay 💖✨