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, Compare comp );  (4)  (since C++17) 
Merges two consecutive sorted ranges [first, middle)
and [middle, last)
into one sorted range [first, last)
. 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>>
is truefirst    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:
The signature does not need to have 
Type requirements  
BidirIt must meet the requirements of ValueSwappable and BidirectionalIterator . 

The type of dereferenced BidirIt must meet the requirements of MoveAssignable and MoveConstructible . 
(none).
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.The overloads with a template parameter named ExecutionPolicy
report errors as follows:
ExecutionPolicy
is one of the three standard policies, std::terminate
is called. For any other ExecutionPolicy
, the behavior is implementationdefined. std::bad_alloc
is thrown. This function attempts to allocate a temporary buffer. If the allocation fails, the less efficient algorithm is chosen.
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
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) 
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