std::rotate(3)

NAME
std::rotate - std::rotate

Synopsis
   Defined in header <algorithm>
   template< class ForwardIt >
   ForwardIt rotate( ForwardIt first, ForwardIt middle, (1) (constexpr since C++20)
   ForwardIt last );
   template< class ExecutionPolicy, class ForwardIt >
   ForwardIt rotate( ExecutionPolicy&& policy, (2) (since C++17)
                     ForwardIt first, ForwardIt middle,
   ForwardIt last );
   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. (since C++11)
     * The type of *first is not MoveAssignable.

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 LegacyForwardIterator.

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

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

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.

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;
   }

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.

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
   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 returned
                      was not returned

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