< cpp‎ | algorithm
Algorithm library
Constrained algorithms and algorithms on ranges (C++20)
Constrained algorithms, e.g. ranges::copy, ranges::sort, ...
Execution policies (C++17)
Non-modifying sequence operations
Batch operations
Search operations
(C++11)                (C++11)(C++11)

Modifying sequence operations
Copy operations
Swap operations
Transformation operations
Generation operations
Removing operations
Order-changing operations
(until C++17)(C++11)
Sampling operations

Sorting and related operations
Partitioning operations
Sorting operations
Binary search operations
(on partitioned ranges)
Set operations (on sorted ranges)
Merge operations (on sorted ranges)
Heap operations
Minimum/maximum operations
Lexicographical comparison operations
Permutation operations
C library
Numeric operations
Operations on uninitialized memory
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 [firstlast) 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

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.


[edit] Parameters

first, last - the 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. See execution policy for details.
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.

[edit] 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.

[edit] 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.

[edit] 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.

[edit] 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_false = *first;
    return std::pair<OutputIt1, OutputIt2>(d_first_true, d_first_false);

[edit] 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);


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

[edit] 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
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

[edit] See also

divides a range of elements into two groups
(function template) [edit]
divides elements into two groups while preserving their relative order
(function template) [edit]
copies a range of elements to a new location
(function template) [edit]
copies a range of elements omitting those that satisfy specific criteria
(function template) [edit]
copies a range dividing the elements into two groups