std::rotate

If any of the following conditions is satisfied, the behavior is undefined:

• [first, middle) or [middle, last) is not a valid range.

[edit] Parameters

[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

#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);
}

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.

[edit] See also