std::rotate

Defined in header `<algorithm>`
	(1)
template< class ForwardIt > ForwardIt rotate( ForwardIt first, ForwardIt middle, ForwardIt last );			(until C++20)
template< class ForwardIt > constexpr ForwardIt rotate( ForwardIt first, ForwardIt middle, ForwardIt last );			(since C++20)
template< class ExecutionPolicy, class ForwardIt > ForwardIt rotate( ExecutionPolicy&& policy, ForwardIt first, ForwardIt middle, ForwardIt last );		(2)	(since C++17)

1) Performs a left rotation on a range of elements.

Specifically, std::rotate swaps the elements in the range [first, last) in such a way that the elements in [first, middle) are placed after the elements in [middle, last) while the orders of the elements in both ranges are preserved.

2) Same as (1), but executed according to policy. This overload does not participate in overload resolution unless

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)

If [first, middle) or [middle, last) is not a valid range, the behavior is undefined.

Parameters

first	-	the beginning of the original range
middle	-	the element that should appear at the beginning of the rotated range
last	-	the end of the original range
policy	-	the execution policy to use. See execution policy for details.
Type requirements
- `ForwardIt` must meet the requirements of ValueSwappable and LegacyForwardIterator.
- The type of dereferenced `ForwardIt` must meet the requirements of MoveAssignable and MoveConstructible.

Return value

An iterator that is equal to:

last, if first == middle is true,
first, if middle == last is true,
first + (last - middle)^[1] otherwise, i.e. the new location of the element pointed by first.

↑ The + and - operations are not required to be supported, they are only used to represent to position of the returned iterator.

Complexity

Linear in the distance between first and last.

Exceptions

The overload with a template parameter named ExecutionPolicy reports 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

std::rotate has better efficiency on common implementations if ForwardIt satisfies LegacyBidirectionalIterator or (better) LegacyRandomAccessIterator.

Implementations (e.g. MSVC STL) may enable vectorization when the iterator type satisfies LegacyContiguousIterator and swapping its value type calls neither non-trivial special member function nor ADL-found swap.

Possible implementation

See also the implementations in libstdc++, libc++, and MSVC STL.

template<class ForwardIt>
constexpr // since C++20
ForwardIt rotate(ForwardIt first, ForwardIt middle, ForwardIt last)
{
    if (first == middle)
        return last;
 
    if (middle == last)
        return first;
 
    ForwardIt write = first;
    ForwardIt next_read = first; // read position for when "read" hits "last"
 
    for (ForwardIt read = middle; read != last; ++write, ++read)
    {
        if (write == next_read)
            next_read = read; // track where "first" went
        std::iter_swap(write, read);
    }
 
    // rotate the remaining sequence into place
    rotate(write, next_read, last);
    return write;
}

Example

std::rotate is a common building block in many algorithms. This example demonstrates insertion sort.

Run this code

#include <algorithm>
#include <iostream>
#include <vector>
 
auto print = [](auto const remark, auto const& v)
{
    std::cout << remark;
    for (auto n : v)
        std::cout << n << ' ';
    std::cout << '\n';
};
 
int main()
{
    std::vector<int> v {2, 4, 2, 0, 5, 10, 7, 3, 7, 1};
    print("before sort:\t\t", v);
 
    // insertion sort
    for (auto i = v.begin(); i != v.end(); ++i)
        std::rotate(std::upper_bound(v.begin(), i, *i), i, i + 1);
    print("after sort:\t\t", v);
 
    // simple rotation to the left
    std::rotate(v.begin(), v.begin() + 1, v.end());
    print("simple rotate left:\t", v);
 
    // simple rotation to the right
    std::rotate(v.rbegin(), v.rbegin() + 1, v.rend());
    print("simple rotate right:\t", v);
}

Output:

before sort:		2 4 2 0 5 10 7 3 7 1
after sort:		0 1 2 2 3 4 5 7 7 10
simple rotate left:	1 2 2 3 4 5 7 7 10 0
simple rotate right:	0 1 2 2 3 4 5 7 7 10

Defect reports

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

DR	Applied to	Behavior as published	Correct behavior
LWG 488	C++98	the new location of the element pointed by first was not returned	returned

Compiler support
Freestanding and hosted
Language
Standard library
Standard library headers
Named requirements
Feature test macros (C++20)
Language support library
Concepts library (C++20)
Metaprogramming library (C++11)
Diagnostics library
General utilities library
Strings library
Containers library
Iterators library
Ranges library (C++20)
Algorithms library
Numerics library
Localizations library
Input/output library
Filesystem library (C++17)
Regular expressions library (C++11)
Concurrency support library (C++11)
Technical specifications
Symbols index
External libraries

rotate_copy	copies and rotate a range of elements (function template)
ranges::rotate (C++20)	rotates the order of elements in a range (niebloid)