< cpp‎ | algorithm‎ | ranges
Algorithm library
Constrained algorithms and algorithms on ranges (C++20)
Constrained algorithms, e.g. ranges::copy, ranges::sort, ...
Execution policies (C++17)
Non-modifying sequence operations
Batch operations
Search operations
(C++11)                (C++11)(C++11)

Modifying sequence operations
Copy operations
Swap operations
Transformation operations
Generation operations
Removing operations
Order-changing operations
(until C++17)(C++11)
Sampling operations

Sorting and related operations
Partitioning operations
Sorting operations
Binary search operations
(on partitioned ranges)
Set operations (on sorted ranges)
Merge operations (on sorted ranges)
Heap operations
Minimum/maximum operations
Lexicographical comparison operations
Permutation operations
C library
Numeric operations
Operations on uninitialized memory
Constrained algorithms
Non-modifying sequence operations
Modifying sequence operations
Partitioning operations
Sorting operations
Binary search operations (on sorted ranges)
Set operations (on sorted ranges)
Heap operations
Minimum/maximum operations
Permutation operations
Numeric operations
Fold operations
Operations on uninitialized storage
Return types
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 >
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 );
(1) (since C++23)
template< ranges::input_range R,

                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 );
(2) (since C++23)
Helper concepts
template< class F, class T, class I >
concept __indirectly_binary_left_foldable = /* see description */;
(3) (exposition only*)

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 [firstlast) is not a valid range.

1) The range is [firstlast). Equivalent to return ranges::fold_left_first_with_iter(std::move(first), last, 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:
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::invoke_result_t<F&, U, std::iter_reference_t<I>>>;
(3A) (exposition only*)
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>>>>;
(3B) (exposition only*)

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.


[edit] Parameters

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

[edit] 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.

[edit] 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>
        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,
                 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 (*this)(ranges::begin(r), ranges::end(r), std::ref(f));
inline constexpr fold_left_first_fn fold_left_first;

[edit] Complexity

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

[edit] Notes

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) 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 left first element

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

[edit] 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

[edit] References

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

[edit] See also

left-folds a range of elements
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) [edit]
similar to std::accumulate, except out of order
(function template) [edit]