NAME¶

std::inplace_merge - std::inplace_merge

Synopsis¶

Defined in header <algorithm>
template< class BidirIt >
void inplace_merge( BidirIt first, BidirIt middle, (1) (constexpr since C++26)
BidirIt last );
template< class ExecutionPolicy, class BidirIt >

void inplace_merge( ExecutionPolicy&& policy, (2) (since C++17)

BidirIt first, BidirIt middle,
BidirIt last );
template< class BidirIt, class Compare >

void inplace_merge( BidirIt first, BidirIt middle, (3) (constexpr since C++26)
BidirIt last,

Compare comp );
template< class ExecutionPolicy, class BidirIt, class
Compare >

void inplace_merge( ExecutionPolicy&& policy, (4) (since C++17)
BidirIt first, BidirIt middle,
BidirIt last,

Compare comp );

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

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

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 output range overlaps with [first, middle) or [middle, last).

* The type of *first is not Swappable. (until C++11)
* BiditIt is not ValueSwappable.
* The type of *first is not MoveConstructible. (since C++11)
* The type of *first is not MoveAssignable.

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. See execution policy for details.
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);
comp -
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.

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.

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.

Possible implementation¶

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

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

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';
}

Output:¶

-2 0 1 2 3 7 8 11 11

See also¶

merge merges two sorted ranges
(function template)
sort sorts a range into ascending order
(function template)
sorts a range of elements while preserving order between equal
stable_sort elements
(function template)
ranges::inplace_merge merges two ordered ranges in-place
(C++20) (niebloid)