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std::shift_left, std::shift_right

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

#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 .

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_leftranges::shift_right (C++23)	shifts elements in a range (niebloid)