std::inplace_merge

Since C++17

Header: <algorithm>

Merges two consecutive sorted ranges [first,middle) and [middle,last) into one sorted range [first,last).

# Declarations

template< class BidirIt >
void inplace_merge( BidirIt first, BidirIt middle, BidirIt last );

(constexpr since C++26)

template< class ExecutionPolicy, class BidirIt >
void inplace_merge( ExecutionPolicy&& policy,
BidirIt first, BidirIt middle, BidirIt last );

(since C++17)

template< class BidirIt, class Compare >
void inplace_merge( BidirIt first, BidirIt middle, BidirIt last,
Compare comp );

(constexpr since C++26)

template< class ExecutionPolicy, class BidirIt, class Compare >
void inplace_merge( ExecutionPolicy&& policy,
BidirIt first, BidirIt middle, BidirIt last,
Compare comp );

(since C++17)

# 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
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.

# Notes

This function attempts to allocate a temporary buffer. If the allocation fails, the less efficient algorithm is chosen.

# Example

#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';
}