Left-folds the elements of given range, that is, returns the result of evaluation of the chain expression:
f(f(f(f(x1, x2), x3), ...), xn), wherex1,x2, ...,xn are elements of the range.

Informally,ranges::fold_left_first behaves likestd::accumulate's overload that accepts a binary predicate, except that the*first is used internally as an initial element.

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

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 left-fold of the given range overf, whereU is equivalent todecltype(ranges::fold_left(std::move(first), last,std::iter_value_t<I>(*first), f)).

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

[edit]Possible implementations

struct fold_left_first_fn{template<std::input_iterator I,std::sentinel_for<I> S,/*indirectly-binary-left-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_left(std::move(first), last,std::iter_value_t<I>(*first), f));if(first== last)returnstd::optional<U>();std::optional<U> init(std::in_place,*first);for(++first; first!= last;++first)*init=std::invoke(f, std::move(*init),*first);return std::move(init);} template<ranges::input_range R,/*indirectly-binary-left-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_left_first_fn fold_left_first;

[edit]Complexity

Exactlyranges::distance(first, last)-1 (assuming the range is not empty) applications of the function objectf.

[edit]Notes

The following table compares all constrained folding algorithms:

[edit]Example

#include <algorithm>#include <array>#include <functional>#include <ranges>#include <utility> int main(){constexprstd::array v{1,2,3,4,5,6,7,8};    static_assert(*std::ranges::fold_left_first(v.begin(), v.end(),std::plus{})==36&&*std::ranges::fold_left_first(v,std::multiplies{})==40320); constexprstd::array w{1,2,3,4,13,1,2,3,4,13,1,2,3,4,13,1,2,3,4,};    static_assert("Find the only value that (by precondition) occurs odd number of times:"&&*std::ranges::fold_left_first(w,[](int p,int q){return p^ q;})==13); constexprauto pairs=std::to_array<std::pair<char,float>>({{'A',3.0f},{'B',3.5f},{'C',4.0f}});    static_assert("Get the product of all pair::second in pairs:"&&*std::ranges::fold_left_first(            pairs| std::ranges::views::values,std::multiplies{})==42);}

[edit]References

[edit]See also

• In other languages

• Deutsch
• Español
• Français
• Italiano
• 日本語
• Português
• Русский
• 中文