NAME¶

std::partition_copy - std::partition_copy

Synopsis¶

Defined in header <algorithm>
template< class InputIt, class OutputIt1,

class OutputIt2, class UnaryPred >
std::pair<OutputIt1, OutputIt2> (since C++11)
partition_copy( InputIt first, InputIt last, (1) (constexpr since C++20)
OutputIt1 d_first_true, OutputIt2
d_first_false,

UnaryPred p );
template< class ExecutionPolicy, class ForwardIt1, class
ForwardIt2,

class ForwardIt3, class UnaryPred >
std::pair<ForwardIt2, ForwardIt3>
partition_copy( ExecutionPolicy&& policy, (2) (since C++17)
ForwardIt1 first, ForwardIt1 last,
ForwardIt2 d_first_true, ForwardIt3
d_first_false,

UnaryPred p );

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

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 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.
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
p - 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¶

// Run this code

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

See also¶

partition divides a range of elements into two groups
(function template)
divides elements into two groups while preserving their
stable_partition relative order
(function template)
copy copies a range of elements to a new location
copy_if (function template)
(C++11)
remove_copy copies a range of elements omitting those that satisfy
remove_copy_if specific criteria
(function template)
ranges::partition_copy copies a range dividing the elements into two groups
(C++20) (niebloid)