NAME¶

std::shift_left,std::shift_right - std::shift_left,std::shift_right

Synopsis¶

Defined in header <algorithm>
template< class ForwardIt >

constexpr ForwardIt shift_left( ForwardIt first, ForwardIt last,
typename (1) (since C++20)
std::iterator_traits<ForwardIt>::

difference_type n );
template< class ExecutionPolicy, class ForwardIt >

ForwardIt shift_left( ExecutionPolicy&& policy,
ForwardIt first, ForwardIt last, (2) (since C++20)
typename std::iterator_traits<ForwardIt>::

difference_type n );
template< class ForwardIt >

constexpr ForwardIt shift_right( ForwardIt first, ForwardIt last,
typename (3) (since C++20)
std::iterator_traits<ForwardIt>::

difference_type n );
template< class ExecutionPolicy, class ForwardIt >

ForwardIt shift_right( ExecutionPolicy&& policy,
ForwardIt first, ForwardIt last, (4) (since C++20)
typename std::iterator_traits<ForwardIt>::

difference_type n );

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.
* 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.

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.

Return value¶

1,2) The end of the resulting range.
* If n is less than std::distance(first, last), returns an iterator equal to
std::next(first, (std::distance(first, last) - n)).
* Otherwise, returns first.
3,4) The beginning of the resulting range.
* If n is less than std::distance(first, last), returns an iterator equal to
std::next(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 Feature
__cpp_lib_shift 201806L (C++20) std::shift_left and std::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::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 .

See also¶

move moves a range of elements to a new location
(C++11) (function template)
move_backward moves a range of elements to a new location in backwards order
(C++11) (function template)
rotate rotates the order of elements in a range
(function template)
ranges::shift_left shifts elements in a range
ranges::shift_right (niebloid)
(C++23)