std::partition_copy - cppreference.com (original) (raw)
| Defined in header | ||
|---|---|---|
| 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) |
- 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.
- Same as (1), but executed according to policy.
This overload participates in overload resolution only if all following conditions are satisfied:
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.
Contents
- 1 Parameters
- 2 Return value
- 3 Complexity
- 4 Exceptions
- 5 Possible implementation
- 6 Example
- 7 Defect reports
- 8 See also
[edit] 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. |
[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
ExecutionPolicyis one of the standard policies, std::terminate is called. For any otherExecutionPolicy, 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_true; } else { *d_first_false = *first; ++d_first_false; } } return std::pair<OutputIt1, OutputIt2>(d_first_true, d_first_false); } |
[edit] Example
#include #include #include 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
[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++11C++17 | 1. the value type of InputIt (C++11)/ForwardIt1 (C++17) was required to be CopyAssignable2. the two output ranges could overlap | 1. not required2. the behavior is undefined in this case |