std::ranges::fold_right

template< std::bidirectional_iterator I, std::sentinel_for<I> S, class T,
          __indirectly_binary_right_foldable<T, I> F >
constexpr auto fold_right( I first, S last, T init, F f );

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

template< class F, class T, class I >
concept __indirectly_binary_left_foldable = /* see description */;

template< class F, class T, class I >
concept __indirectly_binary_right_foldable = /* see description */;

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

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>>>;

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>>>>;

template< class F, class T, class I >
concept /*indirectly-binary-right-foldable*/ =
  /*indirectly-binary-left-foldable*/</*flipped*/<F>, T, I>;

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&& );
};

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

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,
             __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,
             __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:

Example

#include <algorithm>
#include <functional>
#include <iostream>
#include <ranges>
#include <string>
#include <utility>
#include <vector>
using namespace std::literals;
 
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(v.begin(), v.end(), 6, std::plus<>()); // (1)
    std::cout << "r1: " << r1 << '\n';
 
    auto r2 = std::ranges::fold_right(vs, "!"s, std::plus<>()); // (2)
    std::cout << "r2: " << r2 << '\n';
 
    // Use a program defined function object (lambda-expression):
    std::string r3 = std::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 = std::ranges::fold_right
    (
        data | std::ranges::views::values, 2.0f, std::multiplies<>()
    );
    std::cout << "r4: " << r4 << '\n';
}

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

References

• C++23 standard (ISO/IEC 14882:2023):
• 27.6.18 Fold [alg.fold]

See also