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std::ranges..._right_last(3) C++ Standard Libary std::ranges..._right_last(3) NAME std::ranges::fold_right_last - std::ranges::fold_right_last Synopsis 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< (1) (since C++23) std::iter_value_t<I>, std::iter_reference_t<I>> constexpr auto fold_right_last( I first, S last, F f ); template< ranges::bidirectional_range R, /*indirectly-binary-right-foldable*/< ranges::range_value_t<R>, ranges::iterator_t<R>> F > requires std::constructible_from< (2) (since C++23) ranges::range_value_t<R>, ranges::range_reference_t<R>> constexpr auto fold_right_last( R&& r, F f ); Helper concepts template< class F, class T, class I > (exposition concept /*indirectly-binary-left-foldable*/ = /* see description (3) only*) */; template< class F, class T, class I > (exposition concept /*indirectly-binary-right-foldable*/ = /* see description (4) 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 de- cltype(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> && (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 > (ex- position 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 call- ing 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 in- hibited. 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 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-fold- able*/<std::iter_value_t<I>, I> F> requires std::constructible_from<std::iter_value_t<I>, std::iter_ref- erence_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 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::bor- rowed_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::bor- rowed_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) { re- turn 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 14882:2023): * 27.6.18 Fold [alg.fold] See also ranges::fold_right right-folds a range of elements (C++23) (niebloid) ranges::fold_left left-folds a range of elements (C++23) (niebloid) ranges::fold_left_first left-folds a range of elements us- ing 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 us- ing 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 ele- ments (function template) reduce similar to std::accumulate, except out of order (C++17) (function template) http://cppreference.com 2024.06.10 std::ranges..._right_last(3)
NAME | Synopsis | Parameters | Return value | Possible implementations | Complexity | Notes | Example | Output: | References | See also
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