std::ranges::fold_left_first_with_iter, std::ranges::fold_left_first_with_iter_result

Defined in header <algorithm>
Call signature
template< std::input_iterator I, std::sentinel_for<I> S,
          __indirectly_binary_left_foldable<std::iter_value_t<I>, I> F >
    std::constructible_from<std::iter_value_t<I>, std::iter_reference_t<I>>
constexpr /* see description */
    fold_left_first_with_iter( I first, S last, F f );
(1) (since C++23)
template< ranges::input_range R,
              ranges::range_value_t<R>, ranges::iterator_t<R>> F >
    std::constructible_from<ranges::range_value_t<R>, ranges::range_reference_t<R>>
constexpr /* see description */
    fold_left_first_with_iter( R&& r, F f );
(2) (since C++23)
Helper concepts
template< class F, class T, class I >
    concept __indirectly_binary_left_foldable =    // exposition only
        /* see description */;
(3) (since C++23)
Helper class template
template< class I, class T >
    using fold_left_first_with_iter_result = ranges::in_value_result<I, T>;
(4) (since C++23)

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_with_iter 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 [firstlast) is not a valid range.

1) The range is [firstlast).
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:
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>>>>;
4) The return type alias. See Return value section for details.

The function-like entities described on this page are niebloids, that is:

In practice, they may be implemented as function objects, or with special compiler extensions.


first, last - the range of elements to fold
r - the range of elements to fold
f - the binary function object

Return value

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

1) An object of type ranges::fold_left_first_with_iter_result<I, std::optional<U>>.
  • The member ranges::in_value_result::in holds an iterator to the end of the range.
  • The member ranges::in_value_result::value holds the result of the left-fold of given range over f.
If the range is empty, the return value is {std::move(first), std::optional<U>()}.
2) Same as (1) except that the return type is ranges::fold_left_first_with_iter_result<ranges::borrowed_iterator_t<R>, std::optional<U>>.

Possible implementations

class fold_left_first_with_iter_fn
    template<class O, class I, class S, class F>
    constexpr auto impl(I&& first, S&& last, F f) const
        using U = decltype(
            ranges::fold_left(std::move(first), last, std::iter_value_t<I>(*first), f)
        using Ret = ranges::fold_left_first_with_iter_result<O, std::optional<U>>;
        if (first == last)
            return Ret{std::move(first), 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 Ret{std::move(first), std::move(init)};
    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
        return impl<I>(std::move(first), std::move(last), std::ref(f));
    template<ranges::input_range R, __indirectly_binary_left_foldable<
        ranges::range_value_t<R>, ranges::iterator_t<R>> F>
        std::constructible_from<ranges::range_value_t<R>, ranges::range_reference_t<R>>
    constexpr auto operator()(R&& r, F f) const
        return impl<ranges::borrowed_iterator_t<R>>(
            ranges::begin(r), ranges::end(r), std::ref(f)
inline constexpr fold_left_first_with_iter_fn fold_left_first_with_iter;


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:

Fold function template Starts from Initial value Return type
ranges::fold_left left init U
ranges::fold_left_first left first element std::optional<U>
ranges::fold_right right init U
ranges::fold_right_last right last element std::optional<U>
ranges::fold_left_with_iter left init

(1) std::in_value_result<I, U>

(2) std::in_value_result<BR, U>,

where BR is ranges::borrowed_iterator_t<R>

ranges::fold_left_first_with_iter left first element

(1) std::in_value_result<I, std::optional<U>>

(2) std::in_value_result<BR, std::optional<U>>

where BR is ranges::borrowed_iterator_t<R>

Feature-test macro Value Std Comment
__cpp_lib_ranges_fold 202207L (C++23) std::ranges fold algorithms


#include <algorithm>
#include <cassert>
#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_with_iter
        v.begin(), v.end(), std::plus<int>()
    std::cout << "sum: " << sum.value.value() << '\n';
    assert(sum.in == v.end());
    auto mul = std::ranges::fold_left_first_with_iter(v, std::multiplies<int>());
    std::cout << "mul: " << mul.value.value() << '\n';
    assert(mul.in == v.end());
    // 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', 7.f}};
    auto sec = std::ranges::fold_left_first_with_iter
        data | std::ranges::views::values, std::multiplies<>()
    std::cout << "sec: " << sec.value.value() << '\n';
    // use a program defined function object (lambda-expression):
    auto lambda = [](int x, int y) { return x + y + 2; };
    auto val = std::ranges::fold_left_first_with_iter(v, lambda);
    std::cout << "val: " << val.value.value() << '\n';
    assert(val.in == v.end());


sum: 36
mul: 40320
sec: 42
val: 50


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

See also

left-folds a range of elements
left-folds a range of elements using the first element as an initial value
right-folds a range of elements
right-folds a range of elements using the last element as an initial value
left-folds a range of elements, and returns a pair (iterator, value)
sums up or folds a range of elements
(function template)
similar to std::accumulate, except out of order
(function template)

© cppreference.com
Licensed under the Creative Commons Attribution-ShareAlike Unported License v3.0.