Defined in header <algorithm>
Call signature
template< std::input_iterator I, std::sentinel_for<I> S, class T,
          __indirectly_binary_left_foldable<T, I> F >
constexpr auto fold_left( I first, S last, T init, F f );
(1) (since C++23)
template< ranges::input_range R, class T,
          __indirectly_binary_left_foldable<T, ranges::iterator_t<R>> F >
constexpr auto fold_left( R&& r, T init, 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)

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

Informally, ranges::fold_left behaves like std::accumulate's overload that accepts a binary predicate.

The behavior is undefined if [firstlast) is not a valid range.

1) The range is [firstlast). Equivalent to return ranges::fold_left_with_iter(std::move(first), last, std::move(init), 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.
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>>>>;

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
init - the initial value of the fold
f - the binary function object

Return value

An object of type U that contains the result of left-fold of the given range over f, where U is equivalent to std::decay_t<std::invoke_result_t<F&, T, std::iter_reference_t<I>>>.

If the range is empty, U(std::move(init)) is returned.

Possible implementations

struct fold_left_fn
    template<std::input_iterator I, std::sentinel_for<I> S, class T,
             __indirectly_binary_left_foldable<T, I> F>
    constexpr auto operator()( I first, S last, T init, F f ) const
        using U = std::decay_t<std::invoke_result_t<F&, T, std::iter_reference_t<I>>>;
        if (first == last)
            return U(std::move(init));
        U accum = std::invoke(f, std::move(init), *first);
        for (++first; first != last; ++first)
            accum = std::invoke(f, std::move(accum), *first);
        return std::move(accum);
    template<ranges::input_range R, class T,
             __indirectly_binary_left_foldable<T, ranges::iterator_t<R>> F>
    constexpr auto operator()( R&& r, T init, F f ) const
        return (*this)(ranges::begin(r), ranges::end(r), std::move(init), std::ref(f));
inline constexpr fold_left_fn fold_left;


Exactly ranges::distance(first, last) 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 <functional>
#include <iostream>
#include <ranges>
#include <string>
#include <utility>
#include <vector>
int main()
    std::vector<int> v {1, 2, 3, 4, 5, 6, 7, 8};
    int sum = std::ranges::fold_left(v.begin(), v.end(), 0, std::plus<int>()); // (1)
    std::cout << "sum: " << sum << '\n';
    int mul = std::ranges::fold_left(v, 1, std::multiplies<int>()); // (2)
    std::cout << "mul: " << mul << '\n';
    // 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', 3.5f}};
    float sec = std::ranges::fold_left
        data | std::ranges::views::values, 2.0f, std::multiplies<>()
    std::cout << "sec: " << sec << '\n';
    // use a program defined function object (lambda-expression):
    std::string str = std::ranges::fold_left
        v, "A", [](std::string s, int x) { return s + ':' + std::to_string(x); }
    std::cout << "str: " << str << '\n';


sum: 36
mul: 40320
sec: 42
str: A:1:2:3:4:5:6:7:8


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

See also

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

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