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_with_iter 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

LetU bedecltype(ranges::fold_left(std::move(first), last,std::iter_value_t<I>(*first), f)).

[edit]Possible implementations

class fold_left_first_with_iter_fn{template<class O,class I,class S,class F>constexprauto impl(I&& first, S&& last, F f)const{using U= decltype(ranges::fold_left(std::move(first), last,std::iter_value_t<I>(*first), f));using Ret= ranges::fold_left_first_with_iter_result<O,std::optional<U>>;if(first== last)return Ret{std::move(first),std::optional<U>()};std::optional<U> init(std::in_place,*first);for(++first; first!= last;++first)*init=std::invoke(f, std::move(*init),*first);return Ret{std::move(first), std::move(init)};} public: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{return impl<I>(std::move(first), std::move(last),std::ref(f));} 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 impl<ranges::borrowed_iterator_t<R>>(ranges::begin(r),ranges::end(r),std::ref(f));}}; inlineconstexpr fold_left_first_with_iter_fn fold_left_first_with_iter;

[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 <cassert>#include <functional>#include <iostream>#include <ranges>#include <utility>#include <vector> int main(){std::vector v{1,2,3,4,5,6,7,8}; auto sum= std::ranges::fold_left_first_with_iter(        v.begin(), v.end(),std::plus<int>());std::cout<<"sum: "<< sum.value.value()<<'\n';assert(sum.in== v.end()); auto mul= std::ranges::fold_left_first_with_iter(v,std::multiplies<int>());std::cout<<"mul: "<< mul.value.value()<<'\n';assert(mul.in== v.end()); // 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',7.f}};auto sec= std::ranges::fold_left_first_with_iter(        data| std::ranges::views::values,std::multiplies<>());std::cout<<"sec: "<< sec.value.value()<<'\n'; // use a program defined function object (lambda-expression):auto lambda=[](int x,int y){return x+ y+2;};auto val= std::ranges::fold_left_first_with_iter(v, lambda);std::cout<<"val: "<< val.value.value()<<'\n';assert(val.in== v.end());}

sum: 36mul: 40320sec: 42val: 50

[edit]References

[edit]See also

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