std::shift_left, std::shift_right
From cppreference.com
| Defined in header <algorithm>
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||
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. - Otherwise, for every integer
iin[0,last - first - n), moves the element originally at positionfirst + n + ito positionfirst + 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
iin[0,last - first - n), moves the element originally at positionfirst + ito positionfirst + 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 >= 0is nottrue.- The type of
*firstis not MoveAssignable. - For
shift_right,ForwardItis neither LegacyBidirectionalIterator nor ValueSwappable.
Parameters
| first, last | - | the pair of iterators defining the range of elements to shift |
| n | - | the number of positions to shift |
| policy | - | the execution policy to use |
| Type requirements | ||
-ForwardIt must meet the requirements of LegacyForwardIterator.
| ||
Return value
1,2) The end of the resulting range.
- If
nis less thanstd::distance(first, last), returns an iterator equal tostd::next(first, (std::distance(first, last) - n)). - Otherwise, returns
first.
3,4) The beginning of the resulting range.
- If
nis less thanstd::distance(first, last), returns an iterator equal tostd::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 (algorithm function object) |