NAME

std::inplace_merge - std::inplace_merge

Synopsis

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

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

Compare comp );

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

A sequence [first, last) is said to be sorted with respect to a comparator comp if
for any iterator it pointing to the sequence and any non-negative integer n such
that it + n is a valid iterator pointing to an element of the sequence, comp(*(it +
n), *it) evaluates to false.

This merge 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).

1) Elements are compared using operator< and the ranges must be sorted with respect
to the same.
3) Elements are compared using the given binary comparison function comp and the
ranges must be sorted with respect to the same.
2,4) Same as (1,3), but executed according to policy. These overloads do not
participate in overload resolution unless
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) is true.

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 ValueSwappable and
LegacyBidirectionalIterator.
-
The type of dereferenced BidirIt must meet the requirements of MoveAssignable and
MoveConstructible.

Return value

(none)

Complexity

Given N = std::distance(first, last)},

1,3) Exactly N-1 comparisons if enough additional memory is available. If the memory
is insufficient, O(N log N) comparisons.
2,4) O(N log N) comparisons.

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.

Notes

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

Possible implementation

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

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