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)