NAME¶

std::ranges::fold_right - std::ranges::fold_right

Synopsis¶

Defined in header <algorithm>
Call signature
template< std::bidirectional_iterator I,
std::sentinel_for<I> S, class T,
(since
/* indirectly-binary-right-foldable */<T, C++23)
I> F > (until
C++26)
constexpr auto fold_right( I first, S last, T init, F
f );
template< std::bidirectional_iterator I,
std::sentinel_for<I> S,

class T = std::iter_value_t<I>, (since
/* indirectly-binary-right-foldable */<T, C++26)
I> F >

constexpr auto fold_right( I first, S last, T init, F
f );
template< ranges::bidirectional_range R, class T,
(1)
/* indirectly-binary-right-foldable */ (since C++23)
<T, ranges::iterator_t<R>> F > (until C++26)

constexpr auto fold_right( R&& r, T init, F f );
template< ranges::bidirectional_range R, class T =
ranges::range_value_t<R>,

/* indirectly-binary-right-foldable */ (2) (since C++26)
<T, ranges::iterator_t<R>> F >

constexpr auto fold_right( R&& r, T init, F f );
Helper concepts
template< class F, class T, class I > (exposition
concept /* indirectly-binary-left-foldable */ = /* (3) only*)
see description */;
template< class F, class T, class I > (exposition
concept /* indirectly-binary-right-foldable */ = /* (4) only*)
see description */;

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], init))), where x[1], x[2], ..., x[n] are elements of the
range.

Informally, ranges::fold_right behaves like std::fold_left(ranges::reverse(r), init,
/* flipped */(f)).

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<U> &&
std::convertible_to<T, U> && (3A) (exposition only*)
std::invocable<F&, U, std::iter_reference_t<I>> &&
std::assignable_from<U&,

std::invoke_result_t<F&, U,
std::iter_reference_t<I>>>;
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, (3B) (exposition only*)
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>>>>;

4) Equivalent to:

Helper concepts
template< class F, class T, class I >
(exposition
concept /*indirectly-binary-right-foldable*/ = (4A) only*)

/*indirectly-binary-left-foldable*/</*flipped*/<F>, T, I>;
Helper class templates
template< class F >

class /*flipped*/
{
F f; // exposition only (exposition
public: (4B) only*)
template< class T, class U >
requires std::invocable<F&, U, T>
std::invoke_result_t<F&, U, T> operator()( T&&, U&& );

};

The function-like entities described on this page are 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.

In practice, they may be implemented as function objects, or with special compiler
extensions.

Parameters¶

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

Return value¶

An object of type U that contains the result of right-fold of the given range over
f, where U is equivalent to std::decay_t<std::invoke_result_t<F&,
std::iter_reference_t<I>, T>>;.

If the range is empty, U(std::move(init)) is returned.

Possible implementations¶

struct fold_right_fn
{
template<std::bidirectional_iterator I, std::sentinel_for<I> S,
class T = std::iter_value_t<I>,
/* indirectly-binary-right-foldable */<T, I> F>
constexpr auto operator()(I first, S last, T init, F f) const
{
using U = std::decay_t<std::invoke_result_t<F&, std::iter_reference_t<I>, T>>;
if (first == last)
return U(std::move(init));
I tail = ranges::next(first, last);
U accum = std::invoke(f, *--tail, std::move(init));
while (first != tail)
accum = invoke(f, *--tail, std::move(accum));
return accum;
}

template<ranges::bidirectional_range R, class T = ranges::range_value_t<R>,
/* indirectly-binary-right-foldable */<T, ranges::iterator_t<R>> F>
constexpr auto operator()(R&& r, T init, F f) const
{
return (*this)(ranges::begin(r), ranges::end(r), std::move(init), std::ref(f));
}
};

inline constexpr fold_right_fn fold_right;

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 Initial Return type
from value
ranges::fold_left left init U
ranges::fold_left_first left first std::optional<U>
element
ranges::fold_right right init U
ranges::fold_right_last right last std::optional<U>
element
(1) ranges::in_value_result<I, U>

ranges::fold_left_with_iter left init (2) ranges::in_value_result<BR, U>,

where BR is
ranges::borrowed_iterator_t<R>
(1) ranges::in_value_result<I,
std::optional<U>>

ranges::fold_left_first_with_iter left first (2) ranges::in_value_result<BR,
element 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 202403 (C++26) List-initialization for
algorithms (1,2)

Example¶

// Run this code

#include <algorithm>
#include <complex>
#include <functional>
#include <iostream>
#include <ranges>
#include <string>
#include <utility>
#include <vector>

using namespace std::literals;
namespace ranges = std::ranges;

int main()
{
auto v = {1, 2, 3, 4, 5, 6, 7, 8};
std::vector<std::string> vs{"A", "B", "C", "D"};

auto r1 = ranges::fold_right(v.begin(), v.end(), 6, std::plus<>()); // (1)
std::cout << "r1: " << r1 << '\n';

auto r2 = ranges::fold_right(vs, "!"s, std::plus<>()); // (2)
std::cout << "r2: " << r2 << '\n';

// Use a program defined function object (lambda-expression):
std::string r3 = ranges::fold_right
(
v, "A", [](int x, std::string s) { return s + ':' + std::to_string(x); }
);
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', 2.f}, {'B', 3.f}, {'C', 3.5f}};
float r4 = ranges::fold_right
(
data | ranges::views::values, 2.0f, std::multiplies<>()
);
std::cout << "r4: " << r4 << '\n';

using CD = std::complex<double>;
std::vector<CD> nums{{1, 1}, {2, 0}, {3, 0}};
#ifdef __cpp_lib_algorithm_default_value_type
auto r5 = ranges::fold_right(nums, {7, 0}, std::multiplies{});
#else
auto r5 = ranges::fold_right(nums, CD{7, 0}, std::multiplies{});
#endif
std::cout << "r5: " << r5 << '\n';
}

Output:¶

r1: 42
r2: ABCD!
r3: A:8:7:6:5:4:3:2:1
r4: 42
r5: (42,42)

References¶

* C++23 standard (ISO/IEC 14882:2023):

* 27.6.18 Fold [alg.fold]

See also¶

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