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

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 niebloids, that is:

Explicit template argument lists may not be specified when calling any of them.
None of them is visible to argument-dependent lookup.
When one of them is found by normal unqualified lookup for the name to the left of the function-call operator, it inhibits argument-dependent lookup.

In practice, they may be implemented as function objects, or with special compiler extensions.

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 testing macro: __cpp_lib_shift.

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 .

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

Name	C++
Version
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Last Updated	2022-06-02T22:02:33Z