std::ranges::fold_left_with_iter, std::ranges::fold_left_with_iter_result

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), ...), x_n), where x₁, x₂, ..., x_n are elements of the range.

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

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

1) The range is [first, last).

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 &&
std::convertible_to <T, U> &&
std::invocable <F&, U, std::iter_reference_t > &&
std::assignable_from <U&,

std::invoke_result_t <F&, U, std::iter_reference_t >>;

(3A) (exposition only*)

template< class F, class T, class I >

concept /*indirectly-binary-left-foldable*/ =
std::copy_constructible <F> &&
std::indirectly_readable &&
std::invocable <F&, T, std::iter_reference_t > &&
std::convertible_to <std::invoke_result_t <F&, T, std::iter_reference_t >,
std::decay_t <std::invoke_result_t <F&, T, std::iter_reference_t >>> &&
/*indirectly-binary-left-foldable-impl*/<F, T, I,

std::decay_t <std::invoke_result_t <F&, T, std::iter_reference_t >>>;

(3B) (exposition only*)

4) The return type alias. See "Return value" section for details.

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.

class fold_left_with_iter_fn
{
 template<class O, class I, class S, class T, class F>
 constexpr auto impl(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>>>;
 using Ret = ranges::fold_left_with_iter_result<O, U>;
 if (first == last)
 return Ret{std::move(first), 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 Ret{std::move(first), std::move(accum)};
 }
public:
 template<std::input_iterator I, std::sentinel_for <I> S, class T = std::iter_value_t <I>,
 /* indirectly-binary-left-foldable */<T, I> F>
 constexpr auto operator()(I first, S last, T init, F f) const
 {
 return impl<I>(std::move(first), std::move(last), std::move(init), std::ref (f));
 }
 
 template<ranges::input_range R, class T = ranges::range_value_t <R>,
 /* indirectly-binary-left-foldable */<T, ranges::iterator_t <R>> F>
 constexpr auto operator()(R&& r, T init, F f) const
 {
 return impl<ranges::borrowed_iterator_t <R>>
 (
 ranges::begin (r), ranges::end (r), std::move(init), std::ref (f)
 );
 }
};
 
inline constexpr fold_left_with_iter_fn fold_left_with_iter;

[edit] Complexity

Exactly ranges::distance (first, last) 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
`__cpp_lib_algorithm_default_value_type`	`202403L`	(C++26)	List-initialization for algorithms (1,2)

[edit] Example

Run this code

#include <algorithm>
#include <cassert>
#include <complex>
#include <functional>
#include <ranges>
#include <utility>
#include <vector>
 
int main()
{
 namespace ranges = std::ranges;
 
 std::vector v{1, 2, 3, 4, 5, 6, 7, 8};
 
 auto sum = ranges::fold_left_with_iter(v.begin(), v.end(), 6, std::plus <int>());
 assert (sum.value == 42);
 assert (sum.in == v.end());
 
 auto mul = ranges::fold_left_with_iter(v, 0X69, std::multiplies <int>());
 assert (mul.value == 4233600);
 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', 3.5f}};
 auto sec = ranges::fold_left_with_iter
 (
 data | ranges::views::values, 2.0f, std::multiplies <>()
 );
 assert (sec.value == 42);
 
 // Use a program defined function object (lambda-expression):
 auto lambda = [](int x, int y){ return x + 0B110 + y; };
 auto val = ranges::fold_left_with_iter(v, -42, lambda);
 assert (val.value == 42);
 assert (val.in == v.end());
 
 using CD = std::complex <double>;
 std::vector <CD> nums{{1, 1}, {2, 0}, {3, 0}};
 #ifdef __cpp_lib_algorithm_default_value_type
 auto res = ranges::fold_left_with_iter(nums, {7, 0}, std::multiplies {});
 #else
 auto res = ranges::fold_left_with_iter(nums, CD{7, 0}, std::multiplies {});
 #endif
 assert ((res.value == CD{42, 42}));
}