Home Original page

std::partition_copy - cppreference.com

From cppreference.com

Defined in header <algorithm>

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

 (1) (since C++11)
(constexpr since C++20)

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

 (2) (since C++17)

1) Copies the elements from the range [first, last) to two different ranges depending on the value returned by the predicate p.

  • The elements that satisfy the predicate p are copied to the range beginning at d_first_true.
  • The rest of the elements are copied to the range beginning at d_first_false.

2) Same as (1), but executed according to policy.

This overload participates in overload resolution only if all following conditions are satisfied:

std::is_execution_policy_v<std::decay_t<ExecutionPolicy>> is true.

(until C++20)

std::is_execution_policy_v<std::remove_cvref_t<ExecutionPolicy>> is true.

(since C++20)

If *first is not writable to d_first_true or d_first_false, the program is ill-formed.

Among the input range and the two output ranges, if any two ranges overlap, the behavior is undefined.

Parameters

first, last - the pair of iterators defining the source range of elements to copy from
d_first_true - the beginning of the output range for the elements that satisfy p
d_first_false - the beginning of the output range for the elements that do not satisfy p
policy - the execution policy to use
p - unary predicate which returns ​true if the element should be placed in d_first_true.

The expression p(v) must be convertible to bool for every argument v of type (possibly const) VT, where VT is the value type of InputIt, regardless of value category, and must not modify v. Thus, a parameter type of VT&is not allowed, nor is VT unless for VT a move is equivalent to a copy(since C++11). ​

Type requirements
-InputIt must meet the requirements of LegacyInputIterator.
-OutputIt1, OutputIt2 must meet the requirements of LegacyOutputIterator.
-ForwardIt1, ForwardIt2, ForwardIt3 must meet the requirements of LegacyForwardIterator.
-UnaryPred must meet the requirements of Predicate.

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 (2), 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

partition_copy (1) 
template<class InputIt, class OutputIt1,
         class OutputIt2, class UnaryPred>
constexpr //< since C++20
std::pair<OutputIt1, OutputIt2>
    partition_copy(InputIt first, InputIt last,
                   OutputIt1 d_first_true, OutputIt2 d_first_false,
                   UnaryPred 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, const auto& v)
{
    for (std::cout << rem; const auto& 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);
}

Output: 

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

Defect reports

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

DR Applied to Behavior as published Correct behavior
P0896R4 C++11
C++17 		1. the value type of InputIt (C++11)/ForwardIt1 (C++17)
    was required to be CopyAssignable
2. the two output ranges could overlap 		1. not required
2. the behavior is
    undefined in this case

See also