std::rotate

Defined in header `<algorithm>`
template< class ForwardIt > ForwardIt rotate( ForwardIt first, ForwardIt middle, ForwardIt last );	(1)	(constexpr 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 participates 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)

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)
`ForwardIt` is not ValueSwappable. The type of first is not MoveConstructible. The type of first is not MoveAssignable.	(since C++11)

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 LegacyForwardIterator.

[edit] Return value

The iterator to the element originally referenced by *first, i.e. the std::distance(middle, last)th next iterator of first.

[edit] Complexity

At most std::distance(first, last) swaps.

[edit] 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.

[edit] 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;
}

[edit] 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.

[edit] 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 = [](const auto remark, const auto& 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

[edit] 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

[edit] See also

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

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[edit] Parameters