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std::rotate

Defined in header `<algorithm>`
	(1)
template< class ForwardIt > void rotate( ForwardIt first, ForwardIt n_first, ForwardIt last );		(until C++11)
template< class ForwardIt > ForwardIt rotate( ForwardIt first, ForwardIt n_first, ForwardIt last );		(since C++11) (until C++20)
template< class ForwardIt > constexpr ForwardIt rotate( ForwardIt first, ForwardIt n_first, ForwardIt last );		(since C++20)
template< class ExecutionPolicy, class ForwardIt > ForwardIt rotate( ExecutionPolicy&& policy, ForwardIt first, ForwardIt n_first, 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 element n_first becomes the first element of the new range and n_first - 1 becomes the last element.

A precondition of this function is that [first, n_first) and [n_first, last) are valid ranges.

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>> (until C++20) std::is_execution_policy_v<std::remove_cvref_t<ExecutionPolicy>> (since C++20) is true.

Parameters

first	-	the beginning of the original range
n_first	-	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

(none).	(until C++11)
Iterator to the new location of the element pointed by `first`. Equal to `first + (last - n_first)`.	(since C++11)

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 statisfies 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 // void until C++11
rotate(ForwardIt first, ForwardIt n_first, ForwardIt last)
{
   if(first == n_first) return last;
   if(n_first == last) return first;
 
   ForwardIt read      = n_first;
   ForwardIt write     = first;
   ForwardIt next_read = first; // read position for when "read" hits "last"
 
   while(read != last) {
      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.

#include <vector>
#include <iostream>
#include <algorithm>
 
auto print = [](auto const& remark, auto const& v) {
    std::cout << remark;
    for (int 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

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