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, (1) (since C++20)

typename std::iterator_traits<ForwardIt>::difference_type n );
template< class ExecutionPolicy, class ForwardIt >

ForwardIt shift_left( ExecutionPolicy&& policy, ForwardIt first, (2) (since C++20)
ForwardIt last,

typename std::iterator_traits<ForwardIt>::difference_type n );
template< class ForwardIt >

constexpr ForwardIt shift_right( ForwardIt first, ForwardIt last, (3) (since C++20)

typename std::iterator_traits<ForwardIt>::difference_type n );
template< class ExecutionPolicy, class ForwardIt >

ForwardIt shift_right( ExecutionPolicy&& policy, ForwardIt first, (4) (since C++20)
ForwardIt last,

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. 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: __cpp_lib_shift

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)