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std::ranges::fold_left_first - cppreference.com

Call signature

Helper concepts

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

Left-folds the elements of given range, that is, returns the result of evaluation of the chain expression:
f(f(f(f(x1, x2), x3), ...), xn), where x1, x2, ..., xn are elements of the range.

Informally, 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.

The behavior is undefined if [first, last) is not a valid range.

Equivalent to:

Helper concepts

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.

An object of type std::optional<U> 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<U>() is returned.

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;

Exactly ranges::distance(first, last) - 1 (assuming the range is not empty) applications of the function object f.

The following table compares all constrained folding algorithms:

#include <algorithm>
#include <array>
#include <functional>
#include <ranges>
#include <utility>
 
int main()
{
    constexpr std::array v{1, 2, 3, 4, 5, 6, 7, 8};
    static_assert
    (
        *std::ranges::fold_left_first(v.begin(), v.end(), std::plus{}) == 36
        && *std::ranges::fold_left_first(v, std::multiplies{}) == 40320
    );
 
    constexpr std::array w
    {
        1, 2, 3, 4, 13,
        1, 2, 3, 4, 13,
        1, 2, 3, 4, 13,
        1, 2, 3, 4,
    };
    static_assert
    (
        "Find the only value that (by precondition) occurs odd number of times:"
        && *std::ranges::fold_left_first(w, [](int p, int q){ return p ^ q; }) == 13
    );
 
    constexpr auto pairs = std::to_array<std::pair<char, float>>
    ({
        {'A', 3.0f},
        {'B', 3.5f},
        {'C', 4.0f}
    });
    static_assert
    (
        "Get the product of all pair::second in pairs:"
        && *std::ranges::fold_left_first
        (
            pairs | std::ranges::views::values, std::multiplies{}
        ) == 42
    );
}

• C++23 standard (ISO/IEC 14882:2024):

• 27.6.18 Fold [alg.fold]

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