# std::shift_left, std::shift_right

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< cpp‎ | algorithm

Defined in header `<algorithm>` |  |
template< class ForwardIt > constexpr ForwardIt shift_left( ForwardIt first, ForwardIt last, typename std::iterator_traits <ForwardIt>::difference_type n ); | (1) | (since C++20)
template< class ExecutionPolicy, class ForwardIt > ForwardIt shift_left( ExecutionPolicy&& policy, ForwardIt first, ForwardIt last, typename std::iterator_traits <ForwardIt>::difference_type n ); | (2) | (since C++20)
template< class ForwardIt > constexpr ForwardIt shift_right( ForwardIt first, ForwardIt last, typename std::iterator_traits <ForwardIt>::difference_type n ); | (3) | (since C++20)
template< class ExecutionPolicy, class ForwardIt > ForwardIt shift_right( ExecutionPolicy&& policy, ForwardIt first, ForwardIt last, typename std::iterator_traits <ForwardIt>::difference_type n ); | (4) | (since C++20)

Shifts the elements in the range `[first, last)` by `n` positions.

1) Shifts the elements towards the beginning of the range. If n <= 0 || n >= last - first, there are no effects. Otherwise, for every integer `i` in [0, last - first - n), moves the element originally at position first + n + i to position first + i. The moves are performed in increasing order of `i` starting from ​0​.
3) Shifts the elements towards the end of the range. If n <= 0 || n >= last - first, there are no effects. Otherwise, for every integer `i` in [0, last - first - n), moves the element originally at position first + i to position first + n + i. If `ForwardIt` meets the LegacyBidirectionalIterator requirements, then the moves are performed in decreasing order of `i` starting from last - first - n - 1.
2,4) Same as (1) and (3), respectively, but executed according to `policy` and the moves may be performed in any order. These overloads does not participate in overload resolution unless std::is_execution_policy_v < std::decay_t <ExecutionPolicy>> is true.

Elements that are in the original range but not the new range are left in a valid but unspecified state.

### Parameters

first | - | the beginning of the original range
last | - | the end of the original range
n | - | the number of positions to shift
policy | - | the execution policy to use. See execution policy for details.
Type requirements
- `ForwardIt` must meet the requirements of LegacyForwardIterator.
- `ForwardIt` must meet either the requirements of LegacyBidirectionalIterator or the requirements of ValueSwappable for overloads (3-4).
- The type of dereferenced `ForwardIt` must meet the requirements of MoveAssignable.

### Return value

1-2) The end of the resulting range. If `n` is positive and less than `last - first`, returns `first + (last - first - n)`. Otherwise if `n` is positive, returns `first`. Otherwise, returns `last`.
3-4) The beginning of the resulting range. If `n` is positive and less than `last - first`, returns `first + n`. Otherwise if `n` is positive, returns `last`. Otherwise, returns `first`.

### Complexity

1-2) At most std::distance (first, last) - n assignments.
3-4) At most std::distance (first, last) - n assignment or swaps.

### Exceptions

The overloads with a template parameter named `ExecutionPolicy` report 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.

### See also

move (C++11) | moves a range of elements to a new location (function template)
move_backward (C++11) | moves a range of elements to a new location in backwards order (function template)
rotate | rotates the order of elements in a range (function template)

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