# std::ranges::shift_left, std::ranges::shift_right

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< cpp‎ | algorithm‎ | ranges

Defined in header `<algorithm>` |  |
Call signature |  |
template< std::permutable I, std::sentinel_for <I> S > constexpr ranges::subrange <I> shift_left( I first, S last, std::iter_difference_t <I> n ); | (1) | (since C++23)
template< ranges::forward_range R > requires std::permutable < ranges::iterator_t <R>> constexpr ranges::borrowed_subrange_t <R> shift_left( R&& r, ranges::range_difference_t <R> n ); | (2) | (since C++23)
template< std::permutable I, std::sentinel_for <I> S > constexpr ranges::subrange <I> shift_right( I first, S last, std::iter_difference_t <I> n ); | (3) | (since C++23)
template< ranges::forward_range R > requires std::permutable < ranges::iterator_t <R>> constexpr ranges::borrowed_subrange_t <R> shift_right( R&& r, ranges::range_difference_t <R> n ); | (4) | (since C++23)

Shifts the elements in the range `[`first`,` last`)` or r by `n` positions. The behavior is undefined if `[`first`,` last`)` is not a valid range.

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 `I` models `bidirectional_iterator`, then the moves are performed in decreasing order of `i` starting from last - first - n - 1.

2,4) Same as (1) or (3) respectively, but uses r as the range, as if using ranges::begin (r) as first and ranges::end (r) as last.

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

The function-like entities described on this page are algorithm function objects (informally known as niebloids), that is:

- Explicit template argument lists cannot be specified when calling any of them.
- None of them are visible to argument-dependent lookup.
- When any of them are found by normal unqualified lookup as the name to the left of the function-call operator, argument-dependent lookup is inhibited.

### Parameters

first | - | the beginning of the original range
last | - | the end of the original range
r | - | the range of elements to shift
n | - | the number of positions to shift

### Return value

1,2) {first, /*NEW_LAST*/}, where `NEW_LAST` is the end of the resulting range and equivalent to:

- first + (last - first - n), if `n` is less than last - first;
- first otherwise.

3,4) {/*NEW_FIRST*/, last}, where `NEW_FIRST` is the beginning of the resulting range and equivalent to:

- first + n, if `n` is less than last - first;
- last otherwise.

### Complexity

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

### Notes

`ranges::shift_left` / `ranges::shift_right` has better efficiency on common implementations if `I` models `bidirectional_iterator` or (better) `random_access_iterator`.

Implementations (e.g. MSVC STL) may enable vectorization when the iterator type models `contiguous_iterator` and swapping its value type calls neither non-trivial special member function nor ADL-found `swap`.

Feature-test macro | Value | Std | Feature
--- | --- | --- | ---
`__cpp_lib_shift` | `202202L` | (C++23) | `std::ranges::shift_left` and `std::ranges::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::ranges::shift_left(a, 3);
        std::ranges::shift_left(b, 3);
        std::ranges::shift_left(c, 3);
        std::cout << a << "  " << b << "  " << c << '\n';

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

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

    //  std::ranges::shift_left(a, -3); // UB
    }

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

ranges::move (C++20) | moves a range of elements to a new location (algorithm function object)
ranges::move_backward (C++20) | moves a range of elements to a new location in backwards order (algorithm function object)
ranges::rotate (C++20) | rotates the order of elements in a range (algorithm function object)
shift_leftshift_right (C++20) | shifts elements in a range (function template)

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