std::stable_partition(3)

NAME
std::stable_partition - std::stable_partition

Synopsis
   Defined in header <algorithm>
   template< class BidirIt, class UnaryPred >
   BidirIt stable_partition( BidirIt first, BidirIt last, (1) (constexpr since C++26)
   UnaryPred p );
   template< class ExecutionPolicy, class BidirIt, class
   UnaryPred >
   BidirIt stable_partition( ExecutionPolicy&& policy, (2) (since C++17)
                        BidirIt first, BidirIt last,
   UnaryPred p );
   1) Reorders the elements in the range [first, last) in such a way that all elements
   for which the predicate p returns true precede the elements for which predicate p
   returns false. Relative order of the elements is preserved.
   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 any of the following conditions is satisfied, the behavior is undefined:
     * The type of *first is not Swappable. (until C++11)
     * BidirIt is not ValueSwappable.
     * The type of *first is not MoveConstructible. (since C++11)
     * The type of *first is not MoveAssignable.

Parameters
   first, last - the range of elements to reorder
   policy - the execution policy to use. See execution policy for details.
                  unary predicate which returns true if the element should be ordered
                  before other elements.
                  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 BidirIt,
                  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
   -
   BidirIt must meet the requirements of LegacyBidirectionalIterator.
   -
   UnaryPred must meet the requirements of Predicate.

Return value
   Iterator to the first element of the second group.

Complexity
   Given \(\scriptsize N\)N as std::distance(first, last):
   1) Exactly \(\scriptsize N\)N applications of p.
   \(\scriptsize O(N)\)O(N) swaps if there is enough extra memory, otherwise at most
   \(\scriptsize N \cdot log_{2}(N)\)N⋅log
   2(N) swaps.
   2) \(\scriptsize O(N)\)O(N) applications of p.
   \(\scriptsize N \cdot log(N)\)N⋅log(N) swaps.

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.

Notes
   This function attempts to allocate a temporary buffer. If the allocation fails, the
   less efficient algorithm is chosen.
   Implementations in libc++ and libstdc++ also accept ranges denoted by
   LegacyForwardIterators as an extension.
         Feature-test macro Value Std Feature
   __cpp_lib_constexpr_algorithms 202306L constexpr stable sorting

Example
// Run this code
 #include <algorithm>
 #include <iostream>
 #include <vector>
 int main()
 {
     std::vector<int> v{0, 0, 3, -1, 2, 4, 5, 0, 7};
     std::stable_partition(v.begin(), v.end(), [](int n) { return n > 0; });
     for (int n : v)
         std::cout << n << ' ';
     std::cout << '\n';
 }

Output:
 3 2 4 5 7 0 0 -1 0
   Defect reports
   The following behavior-changing defect reports were applied retroactively to
   previously published C++ standards.
      DR Applied to Behavior as published Correct behavior
                       std::stable_partition was only required to
   LWG 2150 C++98 place one corrected the
                       element satisfying p before one element not requirement
                       satisfying p

See also
   partition divides a range of elements into two groups
                        (function template)
   ranges::stable_partition divides elements into two groups while preserving their
   (C++20) relative order
                        (niebloid)