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Showing content from https://en.cppreference.com/w/cpp/algorithm/../symbol_index/../algorithm/ranges/partition_copy.html below:

std::ranges::partition_copy, std::ranges::partition_copy_result - cppreference.com

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

(3) (since C++20)

1) Copies the elements from the input range [firstlast) 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.

The function-like entities described on this page are algorithm function objects (informally known as niebloids), that is:

[edit] Parameters first, last - the iterator-sentinel pair defining the source range of elements to copy from r - the source 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 [edit] Return value

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

[edit] Complexity

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

[edit] 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 {};
[edit] 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
[edit] See also

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