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    std::inplace_merge

    From cppreference.com
     
    C++
     
    Algorithm library
    Constrained algorithms and algorithms on ranges (C++20)
    Constrained algorithms, e.g. ranges::copy, ranges::sort, ...
    Non-modifying sequence operations    
    Batch operations
    (C++17)
    Search operations
    Modifying sequence operations
    Copy operations
    (C++11)
    (C++11)
    (C++11)
    Swap operations
    Transformation operations
    Generation operations
    Removing operations
    Order-changing operations
    (until C++17)(C++11)
    (C++20)(C++20)
    Sampling operations
    (C++17)

    Sorting and related operations
    Partitioning operations
    (C++11)    
    (C++11)

    Sorting operations
    Binary search operations
    (on partitioned ranges)
    Set operations (on sorted ranges)
    Merge operations (on sorted ranges)
    Heap operations
    Minimum/maximum operations
    (C++11)
    (C++17)
    Lexicographical comparison operations
    (C++20)
    Permutation operations


     
    Defined in header <algorithm> 
    template< class BidirIt >
void inplace_merge( BidirIt first, BidirIt middle, BidirIt last );
    		(1) (constexpr since C++26) 
    template< class BidirIt, class Compare >
void inplace_merge( BidirIt first, BidirIt middle, BidirIt last,
                    Compare comp );
    		(2) (constexpr since C++26) 
    template< class ExecutionPolicy, class BidirIt >
void inplace_merge( ExecutionPolicy&& policy,
                    BidirIt first, BidirIt middle, BidirIt last );
    		(3) (since C++17) 
    template< class ExecutionPolicy, class BidirIt, class Compare >
void inplace_merge( ExecutionPolicy&& policy,
                    BidirIt first, BidirIt middle, BidirIt last,
                    Compare comp );
    		(4) (since C++17)
    See More

    Makes the target range [first, last) sorted by merging its two consecutive sorted subranges [first, middle) and [middle, last).

    For each group of equivalent elements in the sorted target range, any pair of elements from the group has the following order:

    • If the two elements are from the same subrange, the order between them in the target range follows the original order between them in the subrange.
    • Otherwise, the element from the first subrange is always placed before the element from the second subrange.


    1) The order is determined by operator<(until C++20)std::less{}(since C++20).
    If any of the two subranges is not a valid range sorted with respect to operator<(until C++20)std::less{}(since C++20), the behavior is undefined.
    2) The order is determined by comp.
    If any of the two subranges is not a valid range sorted with respect to comp, the behavior is undefined.
    3,4) Same as (1,2), but executed according to policy.
    These overloads participate in overload resolution only if the value of the following expression is true:

    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)

    If any of the following conditions is satisfied, the behavior is undefined:

    (until C++11)
    (since C++11)

    Parameters

    first - the beginning of the first subrange
    middle - the end of the first subrange and the beginning of the second
    last - the end of the second subrange
    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) 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. ​

    policy - the execution policy to use
    Type requirements
    -
    BidirIt must meet the requirements of LegacyBidirectionalIterator.
    -
    Compare must meet the requirements of Compare.

    Complexity

    Given N as std::distance(first, last):

    1) 𝓞(N⋅log(N)) comparisons using operator<(until C++20)std::less{}(since C++20) (or only exactly N-1 comparisons if enough additional memory is available).
    2) 𝓞(N⋅log(N)) applications of the comparison function comp (or only exactly N-1 applications if enough additional memory is available).
    3) 𝓞(N⋅log(N)) comparisons using operator<(until C++20)std::less{}(since C++20).
    4) 𝓞(N⋅log(N)) applications of the comparison function comp.

    Exceptions

    3,4) During the execution process:
    • If the temporary memory resources required for parallelization are not available, std::bad_alloc is thrown.
    • If an uncaught exception is thrown while accessing objects via an algorithm argument, the behavior is determined by the execution policy (for standard policies, std::terminate is invoked).

    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 (1), (2)

    Possible implementation

    See the implementations in libstdc++ and libc++.

    Example

    The following code is an implementation of merge sort.

    Run this code
    #include <array>
#include <algorithm>
#include <print>

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::array v{8, 2, -2, 0, 11, 11, 1, 7, 3};
    std::print("{}\n", v);
    merge_sort(v.begin(), v.end());
    std::print("{}\n", v);
}

    Output: 

    [8, 2, -2, 0, 11, 11, 1, 7, 3]
[-2, 0, 1, 2, 3, 7, 8, 11, 11]

    See also

    (C++20)
    		 merges two ordered ranges in-place
    (algorithm function object)[edit]
    		merges two sorted ranges
    (function template & algorithm function object)[edit]
    (C++20)
    		sorts a range of elements
    (function template & algorithm function object)[edit]
    (C++20)
    		sorts a range of elements while preserving relative order between equivalent elements
    (function template & algorithm function object)[edit]
    (C++20)