std::inplace_merge

1) If [first, middle) or [middle, last) is not sorted with respect to operator<(until C++20)std::less {}(since C++20), the behavior is undefined.

3) If [first, middle) or [middle, last) is not sorted with respect to comp, the behavior is undefined.

2,4) Same as (1,3), but executed according to policy.

These overloads participate in overload resolution only if all following conditions are satisfied:

std::is_execution_policy_v <std::decay_t <ExecutionPolicy>> is true.

(until C++20)

std::is_execution_policy_v <std::remove_cvref_t <ExecutionPolicy>> is true.

(since C++20)

This merge function 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).

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

[first, middle) or [middle, last) is not a valid range.

The type of *first is not Swappable.

(until C++11)

BiditIt is not ValueSwappable.
The type of *first is not MoveConstructible.
The type of *first is not MoveAssignable.

(since C++11)

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.

Type requirements

-BidirIt must meet the requirements of LegacyBidirectionalIterator.

-Compare must meet the requirements of Compare.

[edit] Complexity

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

1) Exactly \(\scriptsize N-1\)N-1 comparisons using operator<(until C++20)std::less {}(since C++20) if enough additional memory is available, \(\scriptsize O(N \cdot \log(N))\)O(N⋅log(N)) comparisons otherwise.

2) \(\scriptsize O(N \cdot \log(N))\)O(N⋅log(N)) comparisons using operator<(until C++20)std::less {}(since C++20).

3) Exactly \(\scriptsize N-1\)N-1 applications of the comparison function comp if enough additional memory is available, \(\scriptsize O(N \cdot \log(N))\)O(N⋅log(N)) applications otherwise.

4) \(\scriptsize O(N \cdot \log(N))\)O(N⋅log(N)) applications of the comparison function comp.

[edit] 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 other ExecutionPolicy, the behavior is implementation-defined.
If the algorithm fails to allocate memory, std::bad_alloc is thrown.

[edit] Possible implementation

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

[edit] 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), (3)

[edit] Example

The following code is an implementation of merge sort.

Run this code

#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';
}