std::partition_copy

template< class InputIt, class OutputIt1,
          class OutputIt2, class UnaryPredicate >
std::pair<OutputIt1, OutputIt2>
    partition_copy( InputIt first, InputIt last,
                    OutputIt1 d_first_true, OutputIt2 d_first_false,
                    UnaryPredicate p );

template< class InputIt, class OutputIt1,
           class OutputIt2, class UnaryPredicate >
constexpr std::pair<OutputIt1, OutputIt2>
    partition_copy( InputIt first, InputIt last,
                    OutputIt1 d_first_true, OutputIt2 d_first_false,
                    UnaryPredicate p );

template< class ExecutionPolicy, class ForwardIt1, class ForwardIt2,
          class ForwardIt3, class UnaryPredicate >
std::pair<ForwardIt2, ForwardIt3>
    partition_copy( ExecutionPolicy&& policy,
                    ForwardIt1 first, ForwardIt1 last,
                    ForwardIt2 d_first_true, ForwardIt3 d_first_false,
                    UnaryPredicate p );

Parameters

Return value

A std::pair constructed from the iterator to the end of the d_first_true range and the iterator to the end of the d_first_false range.

Complexity

Exactly std::distance(first, last) applications of p.

For the overload with an ExecutionPolicy, there may be a performance cost if ForwardIt's value type is not CopyConstructible.

Exceptions

The overload with a template parameter named ExecutionPolicy reports errors as follows:

• If execution of a function invoked as part of the algorithm throws an exception and ExecutionPolicy is one of the standard policies, std::terminate is called. For any other ExecutionPolicy, the behavior is implementation-defined.
• If the algorithm fails to allocate memory, std::bad_alloc is thrown.

Possible implementation

template<class InputIt, class OutputIt1,
         class OutputIt2, class UnaryPredicate>
std::pair<OutputIt1, OutputIt2>
    partition_copy(InputIt first, InputIt last,
                   OutputIt1 d_first_true, OutputIt2 d_first_false,
                   UnaryPredicate p)
{
    for (; first != last; ++first)
    {
        if (p(*first))
        {
            *d_first_true = *first;
            ++d_first_true;
        }
        else
        {
            *d_first_false = *first;
            ++d_first_false;
        }
    }
 
    return std::pair<OutputIt1, OutputIt2>(d_first_true, d_first_false);
}

Example

#include <algorithm>
#include <iostream>
#include <utility>
 
void print(auto rem, auto const& v)
{
    for (std::cout << rem; auto const& x : v)
        std::cout << x << ' ';
    std::cout << '\n';
}
 
int main()
{
    int arr[10] = {0, 1, 2, 3, 4, 5, 6, 7, 8, 9};
    int true_arr[5] = {0};
    int false_arr[5] = {0};
 
    std::partition_copy(std::begin(arr), std::end(arr),
                        std::begin(true_arr), std::begin(false_arr),
                        [](int i) { return 4 < i; });
 
    print("true_arr:  ", true_arr);
    print("false_arr: ", false_arr);
}

true_arr:  5 6 7 8 9
false_arr: 0 1 2 3 4

See also