Right-folds the elements of given range, that is, returns the result of evaluation of the chain expression:
f(x1, f(x2, ...f(xn-1, xn))), wherex1,x2, ...,xn are elements of the range.

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

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

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

The function-like entities described on this page arealgorithm function objects (informally known asniebloids), that is:

• Explicit template argument lists cannot be specified when calling any of them.
• None of them are visible toargument-dependent lookup.
• When any of them are found bynormal unqualified lookup as the name to the left of the function-call operator,argument-dependent lookup is inhibited.

[edit]Parameters

[edit]Return value

An object of typestd::optional<U> that contains the result of right-fold of the given range overf.

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

[edit]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>    requiresstd::constructible_from<std::iter_value_t<I>,std::iter_reference_t<I>>constexprauto 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)returnstd::optional<U>();        I tail=ranges::prev(ranges::next(first, std::move(last)));returnstd::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>    requiresstd::constructible_from<ranges::range_value_t<R>,ranges::range_reference_t<R>>constexprauto operator()(R&& r, F f)const{return(*this)(ranges::begin(r),ranges::end(r),std::ref(f));}}; inlineconstexpr fold_right_last_fn fold_right_last;

[edit]Complexity

Exactlyranges::distance(first, last) applications of the function objectf.

[edit]Notes

The following table compares all constrained folding algorithms:

[edit]Example

#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';}

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

[edit]References

[edit]See also

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