std::shift_left, std::shift_right
From cppreference.com
| Defined in header <algorithm>
|
||
| template< class ForwardIt > constexpr ForwardIt shift_left( ForwardIt first, ForwardIt last, |
(1) | (since C++20) |
| template< class ExecutionPolicy, class ForwardIt > ForwardIt shift_left( ExecutionPolicy&& policy, |
(2) | (since C++20) |
| template< class ForwardIt > constexpr ForwardIt shift_right( ForwardIt first, ForwardIt last, |
(3) | (since C++20) |
| template< class ExecutionPolicy, class ForwardIt > ForwardIt shift_right( ExecutionPolicy&& policy, |
(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.
- 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,ForwardItis 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
ExecutionPolicyis one of the standard policies, std::terminate is called. For any otherExecutionPolicy, 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
| (C++11) |
moves a range of elements to a new location (function template) |
| (C++11) |
moves a range of elements to a new location in backwards order (function template) |
| rotates the order of elements in a range (function template) | |
| shifts elements in a range (niebloid) |