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 > requires 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, /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 /* 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/ = /* see description */ ;	(3)	(exposition only*)
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), ...), x_n), where x₁, x₂, ..., x_n

Informally, ranges::fold_left_first_with_iter behaves like std::accumulate's overload that accepts a binary predicate, except that the *first

The behavior is undefined if [ first , last )

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 <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>>> ;	(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*)

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

Parameters

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
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)};
    }
 
public:
    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>
    requires
        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;

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:

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

Example

Run this code

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

Output:

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

References

C++23 standard (ISO/IEC 14882:2024):

27.6.18 Fold [alg.fold]

Compiler support
Freestanding and hosted
Language
Standard library
Standard library headers
Named requirements
Feature test macros (C++20)
Language support library
Concepts library (C++20)
Diagnostics library
Memory management library
Metaprogramming library (C++11)
General utilities library
Containers library
Iterators library
Ranges library (C++20)
Algorithms library
Strings library
Text processing library
Numerics library
Date and time library
Input/output library
Filesystem library (C++17)
Concurrency support library (C++11)
Execution support library (C++26)
Technical specifications
Symbols index
External libraries

ranges::fold_left (C++23)	left-folds a range of elements (algorithm function object)
ranges::fold_left_first (C++23)	left-folds a range of elements using the first element as an initial value (algorithm function object)
ranges::fold_right (C++23)	right-folds a range of elements (algorithm function object)
ranges::fold_right_last (C++23)	right-folds a range of elements using the last element as an initial value (algorithm function object)
ranges::fold_left_with_iter (C++23)	left-folds a range of elements, and returns a pair (iterator, value) (algorithm function object)
accumulate	sums up or folds a range of elements (function template)
reduce (C++17)	similar to std::accumulate, except out of order (function template)