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std::ranges::partition_copy, std::ranges::partition_copy_result

Defined in header `<algorithm>`
Call signature
template< std::input_iterator I, std::sentinel_for<I> S, std::weakly_incrementable O1, std::weakly_incrementable O2, class Proj = std::identity, std::indirect_unary_predicate<std::projected<I, Proj>> Pred > requires std::indirectly_copyable<I, O1> && std::indirectly_copyable<I, O2> constexpr partition_copy_result<I, O1, O2> partition_copy( I first, S last, O1 out_true, O2 out_false, Pred pred, Proj proj = {} );	(1)	(since C++20)
template< ranges::input_range R, std::weakly_incrementable O1, std::weakly_incrementable O2, class Proj = std::identity, std::indirect_unary_predicate<std::projected<iterator_t<R>, Proj>> Pred > requires std::indirectly_copyable<ranges::iterator_t<R>, O1> && std::indirectly_copyable<ranges::iterator_t<R>, O2> constexpr partition_copy_result<ranges::borrowed_iterator_t<R>, O1, O2> partition_copy( R&& r, O1 out_true, O2 out_false, Pred pred, Proj proj = {} );	(2)	(since C++20)
Helper types
template< class I, class O1, class O2 > using partition_copy_result = ranges::in_out_out_result<I, O1, O2>;	(3)	(since C++20)

1) Copies the elements from the input range [first, last) to two different output ranges depending on the value returned by the predicate pred. The elements that satisfy the predicate pred after projection by proj are copied to the range beginning at out_true. The rest of the elements are copied to the range beginning at out_false. The behavior is undefined if the input range overlaps either of the output ranges.

2) Same as (1), but uses r as the source range, as if using ranges::begin(r) as first, and ranges::end(r) as last.

The function-like entities described on this page are 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.

In practice, they may be implemented as function objects, or with special compiler extensions.

Parameters

first, last	-	the input range of elements to copy from
r	-	the input range of elements to copy from
out_true	-	the beginning of the output range for the elements that satisfy `pred`
out_false	-	the beginning of the output range for the elements that do not satisfy `pred`
pred	-	predicate to apply to the projected elements
proj	-	projection to apply to the elements

Return value

{last, o1, o2}, where o1 and o2 are the ends of the output ranges respectively, after the copying is complete.

Complexity

Exactly ranges::distance(first, last) applications of the corresponding predicate comp and any projection proj.

Possible implementation

struct partition_copy_fn
{
    template<std::input_iterator I, std::sentinel_for<I> S,
             std::weakly_incrementable O1, std::weakly_incrementable O2,
             class Proj = std::identity, std::indirect_unary_predicate<
             std::projected<I, Proj>> Pred>
    requires std::indirectly_copyable<I, O1> && std::indirectly_copyable<I, O2>
    constexpr ranges::partition_copy_result<I, O1, O2>
        operator()(I first, S last, O1 out_true, O2 out_false, Pred pred, Proj proj = {}) const
    {
        for (; first != last; ++first)
            if (!!std::invoke(pred, std::invoke(proj, *first)))
                *out_true = *first, ++out_true;
            else
                *out_false = *first, ++out_false;
        return {std::move(first), std::move(out_true), std::move(out_false)};
    }
 
    template<ranges::input_range R,
             std::weakly_incrementable O1, std::weakly_incrementable O2,
             class Proj = std::identity,
             std::indirect_unary_predicate<std::projected<iterator_t<R>, Proj>> Pred>
    requires std::indirectly_copyable<ranges::iterator_t<R>, O1> &&
             std::indirectly_copyable<ranges::iterator_t<R>, O2>
    constexpr ranges::partition_copy_result<ranges::borrowed_iterator_t<R>, O1, O2>
        operator()(R&& r, O1 out_true, O2 out_false, Pred pred, Proj proj = {}) const
    {
        return (*this)(ranges::begin(r), ranges::end(r), std::move(out_true),
                       std::move(out_false), std::move(pred), std::move(proj));
    }
};
 
inline constexpr partition_copy_fn partition_copy {};

Example

#include <algorithm>
#include <cctype>
#include <iostream>
#include <iterator>
#include <vector>
 
int main()
{
    const auto in = {'N', '3', 'U', 'M', '1', 'B', '4', 'E', '1', '5', 'R', '9'};
 
    std::vector<int> o1(size(in)), o2(size(in));
 
    auto pred = [](char c) { return std::isalpha(c); };
 
    auto ret = std::ranges::partition_copy(in, o1.begin(), o2.begin(), pred);
 
    std::ostream_iterator<char> cout {std::cout, " "};
    std::cout << "in = ";
    std::ranges::copy(in, cout);
    std::cout << "\no1 = ";
    std::copy(o1.begin(), ret.out1, cout);
    std::cout << "\no2 = ";
    std::copy(o2.begin(), ret.out2, cout);
    std::cout << '\n';
}

Output:

in = N 3 U M 1 B 4 E 1 5 R 9
o1 = N U M B E R
o2 = 3 1 4 1 5 9