std::inplace_merge(3)

NAME
std::inplace_merge - std::inplace_merge

Synopsis
   Defined in header <algorithm>
   template< class BidirIt >
   void inplace_merge( BidirIt first, BidirIt middle, (1) (constexpr since C++26)
   BidirIt last );
   template< class ExecutionPolicy, class BidirIt >
   void inplace_merge( ExecutionPolicy&& policy, (2) (since C++17)
                       BidirIt first, BidirIt middle,
   BidirIt last );
   template< class BidirIt, class Compare >
   void inplace_merge( BidirIt first, BidirIt middle, (3) (constexpr since C++26)
   BidirIt last,
                       Compare comp );
   template< class ExecutionPolicy, class BidirIt, class
   Compare >
   void inplace_merge( ExecutionPolicy&& policy, (4) (since C++17)
                       BidirIt first, BidirIt middle,
   BidirIt last,
                       Compare comp );
   Merges two consecutive sorted ranges [first, middle) and [middle, last) into one
   sorted range [first, last).
   1) If [first, middle) or [middle, last) is not sorted with respect to
   operator<
   (until C++20)
   std::less{}
   (since C++20), the behavior is undefined.
   3) If [first, middle) or [middle, last) is not sorted with respect to comp, the
   behavior is undefined.
   2,4) Same as (1,3), but executed according to policy.
   These overloads participate 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)
   This merge function is stable, which means that for equivalent elements in the
   original two ranges, the elements from the first range (preserving their original
   order) precede the elements from the second range (preserving their original order).
   If any of the following conditions is satisfied, the behavior is undefined:
     * [first, middle) or [middle, last) is not a valid range.
     * The output range overlaps with [first, middle) or [middle, last).
     * The type of *first is not Swappable. (until C++11)
     * BiditIt is not ValueSwappable.
     * The type of *first is not MoveConstructible. (since C++11)
     * The type of *first is not MoveAssignable.

Parameters
   first - the beginning of the first sorted range
   middle - the end of the first sorted range and the beginning of the second
   last - the end of the second sorted range
   policy - the execution policy to use. See execution policy for details.
              comparison function object (i.e. an object that satisfies the
              requirements of Compare) which returns true if the first argument is less
              than (i.e. is ordered before) the second.
              The signature of the comparison function should be equivalent to the
              following:
              bool cmp(const Type1& a, const Type2& b);
   comp -
              While the signature does not need to have const&, the function must not
              modify the objects passed to it and must be able to accept all values of
              type (possibly const) Type1 and Type2 regardless of value category (thus,
              Type1& is not allowed
              , nor is Type1 unless for Type1 a move is equivalent to a copy
              (since C++11)).
              The types Type1 and Type2 must be such that an object of type BidirIt can
              be dereferenced and then implicitly converted to both of them.

Type requirements
   -
   BidirIt must meet the requirements of LegacyBidirectionalIterator.
   -
   Compare must meet the requirements of Compare.

Complexity
   Given \(\scriptsize N\)N as std::distance(first, last):
   1) Exactly \(\scriptsize N-1\)N-1 comparisons using
   operator<
   (until C++20)
   std::less{}
   (since C++20) if enough additional memory is available, \(\scriptsize O(N \cdot
   \log(N))\)O(N⋅log(N)) comparisons otherwise.
   2) \(\scriptsize O(N \cdot \log(N))\)O(N⋅log(N)) comparisons using
   operator<
   (until C++20)
   std::less{}
   (since C++20).
   3) Exactly \(\scriptsize N-1\)N-1 applications of the comparison function comp if
   enough additional memory is available, \(\scriptsize O(N \cdot \log(N))\)O(N⋅log(N))
   applications otherwise.
   4) \(\scriptsize O(N \cdot \log(N))\)O(N⋅log(N)) applications of the comparison
   function comp.

Exceptions
   The overloads with a template parameter named ExecutionPolicy report 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
   See the implementations in libstdc++ and libc++.

Notes
   This function attempts to allocate a temporary buffer. If the allocation fails, the
   less efficient algorithm is chosen.
         Feature-test macro Value Std Feature
   __cpp_lib_constexpr_algorithms 202306L (C++26) constexpr inplace merging

Example
   The following code is an implementation of merge sort. // Run this code
 #include <algorithm>
 #include <iostream>
 #include <vector>
 template<class Iter>
 void merge_sort(Iter first, Iter last)
 {
     if (last - first > 1)
     {
         Iter middle = first + (last - first) / 2;
         merge_sort(first, middle);
         merge_sort(middle, last);
         std::inplace_merge(first, middle, last);
     }
 }
 int main()
 {
     std::vector<int> v{8, 2, -2, 0, 11, 11, 1, 7, 3};
     merge_sort(v.begin(), v.end());
     for (const auto& n : v)
         std::cout << n << ' ';
     std::cout << '\n';
 }

Output:
 -2 0 1 2 3 7 8 11 11

See also
   merge merges two sorted ranges
                        (function template)
   sort sorts a range into ascending order
                        (function template)
                        sorts a range of elements while preserving order between equal
   stable_sort elements
                        (function template)
   ranges::inplace_merge merges two ordered ranges in-place
   (C++20) (niebloid)