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std::ranges::shift_left, std::ranges::shift_right - cppreference.com

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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:

Parameters

first, last - the iterator-sentinel pair defining the range of elements to shift
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

#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