std::inplace_merge

 

C++

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Feature test macros (C++20)

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Concepts library (C++20)

Metaprogramming library (C++11)

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Strings library

Containers library

Iterators library

Ranges library (C++20)

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Filesystem library (C++17)

Regular expressions library (C++11)

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Execution support library (C++26)

Technical specifications

Symbols index

External libraries

 

Algorithm library

Constrained algorithms and algorithms on ranges (C++20)
Constrained algorithms, e.g. ranges::copy, ranges::sort, ...
Execution policies (C++17)
is_execution_policy (C++17)   execution::seqexecution::parexecution::par_unseqexecution::unseq (C++17)    (C++17)(C++17)(C++20)	execution::sequenced_policyexecution::parallel_policyexecution::parallel_unsequenced_policyexecution::parallel_unsequenced (C++17)(C++17)(C++17)(C++20)
Non-modifying sequence operations Batch operations for_each for_each_n (C++17) Search operations all_ofany_ofnone_of (C++11)                (C++11)(C++11) countcount_if mismatch equal findfind_iffind_if_not (C++11) find_end find_first_of adjacent_find search search_n Modifying sequence operations Copy operations copycopy_if (C++11) copy_backward copy_n (C++11) move (C++11) move_backward (C++11) Swap operations swap iter_swap swap_ranges Transformation operations replacereplace_if transform replace_copyreplace_copy_if Generation operations fill fill_n generate generate_n Removing operations removeremove_if unique remove_copyremove_copy_if unique_copy Order-changing operations reverse reverse_copy rotate rotate_copy random_shuffleshuffle (until C++17)(C++11) shift_leftshift_right (C++20)(C++20) Sampling operations sample (C++17)	Sorting and related operations Partitioning operations partition partition_copy (C++11)   stable_partition is_partitioned (C++11) partition_point (C++11) Sorting operations sort stable_sort partial_sort partial_sort_copy is_sorted (C++11) is_sorted_until (C++11) nth_element Binary search operations (on partitioned ranges) lower_bound upper_bound equal_range binary_search Set operations (on sorted ranges) includes set_union set_intersection set_difference set_symmetric_difference Merge operations (on sorted ranges) merge inplace_merge Heap operations push_heap pop_heap make_heap sort_heap is_heap (C++11) is_heap_until (C++11) Minimum/maximum operations max min minmax (C++11) clamp (C++17) max_element min_element minmax_element (C++11) Lexicographical comparison operations lexicographical_compare lexicographical_compare_three_way (C++20) Permutation operations next_permutation prev_permutation is_permutation (C++11) C library qsort bsearch
Numeric operations
iota (C++11) inner_product adjacent_difference    accumulate reduce (C++17) transform_reduce (C++17)	partial_sum inclusive_scan (C++17)    exclusive_scan (C++17) transform_inclusive_scan (C++17) transform_exclusive_scan (C++17)
Operations on uninitialized memory
uninitialized_copy uninitialized_move (C++17)    uninitialized_fill uninitialized_copy_n (C++11) uninitialized_move_n (C++17) uninitialized_fill_n	destroy (C++17) destroy_n (C++17) destroy_at (C++17) construct_at (C++20)       uninitialized_default_construct (C++17) uninitialized_value_construct (C++17) uninitialized_default_construct_n (C++17) uninitialized_value_construct_n (C++17)

 

Defined in header <algorithm>
template< class BidirIt > void inplace_merge( BidirIt first, BidirIt middle, BidirIt last );	(1)	(constexpr since C++26)
template< class ExecutionPolicy, class BidirIt > void inplace_merge( ExecutionPolicy&& policy,                     BidirIt first, BidirIt middle, BidirIt last );	(2)	(since C++17)
template< class BidirIt, class Compare > void inplace_merge( BidirIt first, BidirIt middle, BidirIt last,                     Compare comp );	(3)	(constexpr since C++26)
template< class ExecutionPolicy, class BidirIt, class Compare > void inplace_merge( ExecutionPolicy&& policy,                     BidirIt first, BidirIt middle, BidirIt last,                     Compare comp );	(4)	(since C++17)

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. The type of *first is not MoveAssignable.	(since C++11)

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