# std::rotate

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< cpp‎ | algorithm

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.

1. ↑ 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

### See also

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

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