std::ranges::fold_left_first_with_iter, std::ranges::fold_left_first_with_iter_result
From cppreference.com
| Defined in header <algorithm>
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| Call signature |
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template< std::input_iterator I, std::sentinel_for<I> S,
/*indirectly-binary-left-foldable*/<std::iter_value_t<I>, I> F >
requires std::constructible_from<std::iter_value_t<I>,
std::iter_reference_t<I>>
constexpr ranges::fold_left_first_with_iter_result
<I, std::optional</* see below */>>
fold_left_first_with_iter( I first, S last, F f );
|
(1) | (since C++23) |
template< ranges::input_range R,
/*indirectly-binary-left-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 ranges::fold_left_first_with_iter_result
<ranges::borrowed_iterator_t<R>, std::optional</* see below */>>
fold_left_first_with_iter( R&& r, F f );
|
(2) | (since C++23) |
| Helper template |
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template< class I, class T >
using fold_left_first_with_iter_result = ranges::in_value_result<I, T>;
|
(3) | (since C++23) |
For the definition of /*indirectly-binary-left-foldable*/, see this page.
Left-folds the elements of the source range, that is, returns the result of evaluation of the chain expression:f(f(f(f(x1, x2), x3), ...), xn), where x1, x2, ..., xn are elements of the source range. Also returns the past-the-end iterator of the source range.
1) The source range is
[first, last).2) The source range is
r.3) The return type alias template.
The function-like entities described on this page are algorithm function objects (informally known as 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.
Parameters
| first, last | - | the iterator-sentinel pair defining the source range |
| r | - | the source range |
| f | - | the binary function object |
Return value
A ranges::fold_left_first_with_iter_result object where:
- The data member
inholds the past-the-end iterator of the source range. - The data member
valueholds an std::optional object containing the left-fold result, or does not contain a value if the source range is empty.
The value type of the std::optional in the return type is the decayed type of F's result type.
Notes
The following table compares all constrained folding algorithms. U below is the fold result type, it is the decayed type of F's result type.
| Fold function template | Starts from | Initial value | Return type |
|---|---|---|---|
ranges::fold_left |
left | init |
U
|
ranges::fold_left_first |
left | first element | std::optional<U>
|
ranges::fold_right |
right | init |
U
|
ranges::fold_right_last |
right | last element | std::optional<U>
|
ranges::fold_left_with_iter |
left | init |
(1) (2) where |
ranges::fold_left_first_with_iter |
left | first element |
(1) (2) where |
| Feature-test macro | Value | Std | Feature |
|---|---|---|---|
__cpp_lib_ranges_fold |
202207L |
(C++23) | std::ranges fold algorithms
|
Possible implementations
class fold_left_first_with_iter_fn
{
template<class O, class I, class S, class F>
constexpr auto 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>
requires std::constructible_from<std::iter_value_t<I>,
std::iter_reference_t<I>>
constexpr auto operator()(I first, S last, F f) const
{
return impl<I>(std::move(first), std::move(last), std::ref(f));
}
using BR = ranges::borrowed_iterator_t<R>;
template<ranges::input_range R,
/*indirectly-binary-left-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 impl<BR>(ranges::begin(r), ranges::end(r), std::ref(f));
}
template<ranges::forward_range R,
/*indirectly-binary-left-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 impl<BR>(ranges::begin(r),
ranges::next(ranges::begin(r), ranges::end(r)),
std::ref(f));
}
};
inline constexpr fold_left_first_with_iter_fn fold_left_first_with_iter;
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Example
Run this code
#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());
}
Output:
sum: 36
mul: 40320
sec: 42
val: 50
References
- C++23 standard (ISO/IEC 14882:2024):
- 27.6.18 Fold [alg.fold]
See also
(C++23) |
left-folds a range of elements (algorithm function object) |
(C++23) |
left-folds a range of elements using the first element as an initial value (algorithm function object) |
(C++23) |
right-folds a range of elements (algorithm function object) |
(C++23) |
right-folds a range of elements using the last element as an initial value (algorithm function object) |
(C++23) |
left-folds a range of elements, and returns a pair (iterator, value) (algorithm function object) |
| sums up or folds a range of elements (function template) | |
(C++17) |
similar to std::accumulate, except out of order (function template) |