std::ranges::fold_right_last

Defined in header `<algorithm>`
Call signature
template < std::bidirectional_iterator I, std::sentinel_for <I> S, /indirectly-binary-right-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_right_last( I first, S last, F f ) ;	(1)	(since C++23)
template < ranges::bidirectional_range R, /indirectly-binary-right-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 fold_right_last( 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*)
template < class F, class T, class I > concept /indirectly-binary-right-foldable/ = /* see description */ ;	(4)	(exposition only*)

Right-folds the elements of given range, that is, returns the result of evaluation of the chain expression:
f(x₁, f(x₂, ...f(x_n-1, x_n))), where x₁, x₂, ..., x_n

Informally, ranges::fold_right_last behaves like std:: fold_left ( ranges::reverse (r), *--last, /*flipped*/ (f) )

The behavior is undefined if [ first , last )

1) The range is [first, last). Given U as decltype( ranges::fold_right (first, last, std::iter_value_t <I> ( *first), f) ) , equivalent to:

if (first == last)
    return std::optional<U>();
I tail = ranges::prev(ranges::next(first, std::move(last)));
return std::optional<U>(std::in_place, ranges::fold_right(std::move(first), tail,
    std::iter_value_t<I>(*tail), std::move(f)));

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) Equivalent to:

Helper concepts
template < class F, class T, class I > concept /indirectly-binary-right-foldable/ = /indirectly-binary-left-foldable/ < /flipped/ <F>, T, I> ;	(4A)	(exposition only*)
Helper class templates
template < class F > class /flipped/ { F f; // exposition only public : template < class T, class U > requires std::invocable <F&, U, T> std::invoke_result_t <F&, U, T> operator( ) ( T&&, U&& ) ; } ;	(4B)	(exposition only*)

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

An object of type std::optional<U> that contains the result of right-fold of the given range over f

If the range is empty, std::optional <U> ( )

Possible implementations

struct fold_right_last_fn
{
    template<std::bidirectional_iterator I, std::sentinel_for<I> S,
             /*indirectly-binary-right-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
    {
        using U = decltype(
            ranges::fold_right(first, last, std::iter_value_t<I>(*first), f));
 
        if (first == last)
            return std::optional<U>();
        I tail = ranges::prev(ranges::next(first, std::move(last)));
        return std::optional<U>(std::in_place,
            ranges::fold_right(std::move(first), tail, std::iter_value_t<I>(*tail),
                               std::move(f)));
    }
 
    template<ranges::bidirectional_range R,
             /*indirectly_binary_right_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 (*this)(ranges::begin(r), ranges::end(r), std::ref(f));
    }
};
 
inline constexpr fold_right_last_fn fold_right_last;

Complexity

Exactly ranges::distance(first, last) 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 <functional>
#include <iostream>
#include <ranges>
#include <utility>
#include <vector>
 
int main()
{
    auto v = {1, 2, 3, 4, 5, 6, 7, 8};
    std::vector<std::string> vs {"A", "B", "C", "D"};
 
    auto r1 = std::ranges::fold_right_last(v.begin(), v.end(), std::plus<>()); // (1)
    std::cout << "*r1: " << *r1 << '\n';
 
    auto r2 = std::ranges::fold_right_last(vs, std::plus<>()); // (2)
    std::cout << "*r2: " << *r2 << '\n';
 
    // Use a program defined function object (lambda-expression):
    auto r3 = std::ranges::fold_right_last(v, [](int x, int y) { return x + y + 99; });
    std::cout << "*r3: " << *r3 << '\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 r4 = std::ranges::fold_right_last
    (
        data | std::ranges::views::values, std::multiplies<>()
    );
    std::cout << "*r4: " << *r4 << '\n';
}

Output:

*r1: 36
*r2: ABCD
*r3: 729
*r4: 42

References

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

27.6.18 Fold [alg.fold]

Compiler support
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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_right (C++23)	right-folds a range of elements (algorithm function object)
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_left_with_iter (C++23)	left-folds a range of elements, and returns a pair (iterator, value) (algorithm function object)
ranges::fold_left_first_with_iter (C++23)	left-folds a range of elements using the first element as an initial value, and returns a pair (iterator, optional) (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)