std::inplace_merge
Defined in header <algorithm>


template< class BidirIt > void inplace_merge( BidirIt first, BidirIt middle, BidirIt last ); 
(1)  
template< class ExecutionPolicy, class BidirIt > void inplace_merge( ExecutionPolicy&& policy, BidirIt first, BidirIt middle, BidirIt last ); 
(2)  (since C++17) 
template< class BidirIt, class Compare> void inplace_merge( BidirIt first, BidirIt middle, BidirIt last, Compare comp ); 
(3)  
template< class ExecutionPolicy, class BidirIt, class Compare> void inplace_merge( ExecutionPolicy&& policy, BidirIt first, BidirIt middle, BidirIt last, 
(4)  (since C++17) 
Merges two consecutive sorted ranges [first, middle)
and [middle, last)
into one sorted range [first, last)
.
A sequence [first, last)
is said to be sorted with respect to a comparator comp
if for any iterator it
pointing to the sequence and any nonnegative integer n
such that it + n
is a valid iterator pointing to an element of the sequence, comp(*(it + n), *it) evaluates to false
.
This merge 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).
operator<
and the ranges must be sorted with respect to the same.comp
and the ranges must be sorted with respect to the same.policy
. These overloads do not participate in overload resolution unless std::is_execution_policy_v<std::decay_t<ExecutionPolicy>> (until C++20) std::is_execution_policy_v<std::remove_cvref_t<ExecutionPolicy>> (since C++20) is true.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. 
comp    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); 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) 
Type requirements  
BidirIt must meet the requirements of ValueSwappable and LegacyBidirectionalIterator.
 
The type of dereferenced BidirIt must meet the requirements of MoveAssignable and MoveConstructible.

Return value
(none)
Complexity
Given N = std::distance(first, last)},
N1
comparisons if enough additional memory is available. If the memory is insufficient, O(N log N)
comparisons.O(N log N)
comparisons.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 otherExecutionPolicy
, the behavior is implementationdefined.  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.
Possible implementation
See the implementations in libstdc++ and libc++.
Example
The following code is an implementation of merge sort.
#include <vector> #include <iostream> #include <algorithm> 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(auto n : v) { std::cout << n << ' '; } std::cout << '\n'; }
Output:
2 0 1 2 3 7 8 11 11
See also
merges two sorted ranges (function template)  
sorts a range into ascending order (function template)  
sorts a range of elements while preserving order between equal elements (function template)  
(C++20) 
merges two ordered ranges inplace (niebloid) 