# 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. If n < 0, the behavior is undefined. 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. If n < 0, the behavior is undefined. 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. This overload participates in overload resolution only if std::is_execution_policy_v<std::remove_cvref_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 less than last - first, returns first + (last - first - n). Otherwise, returns first.
3-4) The beginning of the resulting range. If n is less than last - first, returns first + n. Otherwise, returns last.

### 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.

### Notes

Feature-test macro | Value | Std | Comment
--- | --- | --- | ---
`__cpp_lib_shift` | 201806L | (C++20) | `std::shift_left` and `std::shift_right`

### Example

Run this code

    #include <algorithm>
    #include <iostream>
    #include <string>
    #include <type_traits>
    #include <vector>

    struct S
    {
        int value {0};
        bool specified_state {true};

        S(int v = 0) : value {v} {}
        S(S const& rhs) = default;
        S(S&& rhs) { *this = std::move(rhs); }
        S& operator=(S const& rhs) = default;
        S& operator=(S&& rhs)
        {
            if (this != &rhs)
            {
                value = rhs.value;
                specified_state = rhs.specified_state;
                rhs.specified_state = false;
            }
            return *this;
        }
    };

    template<typename T>
    std::ostream& operator<<(std::ostream& os, std::vector<T> const& v)
    {
        for (const auto& s : v)
        {
            if constexpr (std::is_same_v<T, S>)
                s.specified_state ? os << s.value << ' ' : os << ". ";
            else if constexpr (std::is_same_v<T, std::string>)
                os << (s.empty() ? "." : s) << ' ';
            else
                os << s << ' ';
        }
        return os;
    }

    int main()
    {
        std::cout << std::left;

        std::vector<S>            a {1, 2, 3, 4, 5, 6, 7};
        std::vector<int>          b {1, 2, 3, 4, 5, 6, 7};
        std::vector<std::string>  c {"α", "β", "γ", "δ", "ε", "ζ", "η"};

        std::cout << "vector<S> \tvector<int> \tvector<string>\n";
        std::cout << a << "  " << b << "  " << c << '\n';

        std::shift_left(begin(a), end(a), 3);
        std::shift_left(begin(b), end(b), 3);
        std::shift_left(begin(c), end(c), 3);
        std::cout << a << "  " << b << "  " << c << '\n';

        std::shift_right(begin(a), end(a), 2);
        std::shift_right(begin(b), end(b), 2);
        std::shift_right(begin(c), end(c), 2);
        std::cout << a << "  " << b << "  " << c << '\n';

        std::shift_left(begin(a), end(a), 8);  // has no effect: n >= last - first
        std::shift_left(begin(b), end(b), 8);  // ditto
        std::shift_left(begin(c), end(c), 8);  // ditto
        std::cout << a << "  " << b << "  " << c << '\n';

    //  std::shift_left(begin(a), end(a), -3); // UB, e.g. segfault.)
    }

Possible output:

    vector<S>       vector<int>     vector<string>
    1 2 3 4 5 6 7   1 2 3 4 5 6 7   α β γ δ ε ζ η
    4 5 6 7 . . .   4 5 6 7 5 6 7   δ ε ζ η . . .
    . . 4 5 6 7 .   4 5 4 5 6 7 5   . . δ ε ζ η .
    . . 4 5 6 7 .   4 5 4 5 6 7 5   . . δ ε ζ η .

### 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)
ranges::shift_left ranges::shift_right (C++23) | shifts elements in a range (niebloid)

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