std::ranges:: fold_right_last

From cppreference.com

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(x ₁ , f(x ₂ , ...f(x _n-1 , x _n ))) , where x ₁ , x ₂ , ..., x _n are elements of the range.

Informally, ranges::fold_right_last behaves like std :: fold_left ( ranges:: reverse ( r ) , *-- last, /*flipped*/ ( f ) ) (assuming the range is not empty).

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

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 is inhibited.

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 > ( ) is returned.

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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Standard library
Standard library headers
Named requirements
Feature test macros (C++20)
Language support library
Concepts library (C++20)
Metaprogramming library (C++11)
Diagnostics library
General utilities library
Strings library
Containers library
Iterators library
Ranges library (C++20)
Algorithms library
Numerics library
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Filesystem library (C++17)
Regular expressions library (C++11)
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)