std::ranges::fold_left_first

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 );

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 );

template< class F, class T, class I >
    concept __indirectly_binary_left_foldable =    // exposition only
        /* 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.

template< class F, class T, class I, class U >
    concept __indirectly_binary_left_foldable_impl =  // exposition only
        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 =      // exposition only
        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 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

Return value

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.

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.

Notes

The following table compares all constrained folding algorithms:

Example

#include <algorithm>
#include <functional>
#include <iostream>
#include <ranges>
#include <utility>
#include <vector>
 
int main()
{
    std::vector<int> 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';
}

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

References

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

See also