std::shift_left, std::shift_right

If n == 0 || n >= last - first, there are no effects.
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.
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.

These overloads participate 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.

If any of the following conditions is satisfied, the behavior is undefined:

n >= 0 is not true.
The type of *first is not MoveAssignable.
For shift_right, ForwardIt is neither LegacyBidirectionalIterator nor ValueSwappable.

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.

[edit] Notes

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

[edit] 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 .