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# std::inplace_merge

From cppreference.com
< cpp‎ | algorithm

Defined in header `<algorithm>` |  |
template< class BidirIt > void inplace_merge( BidirIt first, BidirIt middle, BidirIt last ); | (1) |
template< class BidirIt, class Compare> void inplace_merge( BidirIt first, BidirIt middle, BidirIt last, Compare comp ); | (2) |

Merges two consecutive sorted ranges `[first, middle)` and `[middle, last)` into one sorted range `[first, last)`. The order of equal elements is guaranteed to be preserved. The first version uses operator< to compare the elements, the second version uses the given comparison function `comp`.

### [edit] 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
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); The signature does not need to have const &, but the function object must not modify the objects passed to it. 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 `ValueSwappable` and `BidirectionalIterator`.
- The type of dereferenced `BidirIt` must meet the requirements of `MoveAssignable` and `MoveConstructible`.

### [edit] Return value

(none)

### [edit] Complexity

Exactly N-1 comparisons if enough additional memory is available, otherwise N·log(N) where N = std::distance(first, last).

### [edit] Notes

This function attempts to allocate a temporary buffer, typically by calling std::get_temporary_buffer. If the allocation fails, the less efficient algorithm is chosen.

### [edit] Example

The following code is an implementation of merge sort.

Run this code

    #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

### [edit] See also

merge | merges two sorted ranges (function template) [edit]
sort | sorts a range into ascending order (function template) [edit]
stable_sort | sorts a range of elements while preserving order between equal elements (function template) [edit]

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