std::rotate

template< class ForwardIt >
ForwardIt rotate( ForwardIt first, ForwardIt middle, ForwardIt last );

template< class ForwardIt >
constexpr ForwardIt rotate( ForwardIt first,
                            ForwardIt middle, ForwardIt last );

template< class ExecutionPolicy, class ForwardIt >
ForwardIt rotate( ExecutionPolicy&& policy,
                  ForwardIt first, ForwardIt middle, ForwardIt last );

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

Parameters

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

#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);
}

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.

See also