std::shift_left, std::shift_right

 

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is_execution_policy (C++17)   execution::seqexecution::parexecution::par_unseqexecution::unseq (C++17)    (C++17)(C++17)(C++20)	execution::sequenced_policyexecution::parallel_policyexecution::parallel_unsequenced_policyexecution::parallel_unsequenced (C++17)(C++17)(C++17)(C++20)
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uninitialized_copy uninitialized_move (C++17)    uninitialized_fill uninitialized_copy_n (C++11) uninitialized_move_n (C++17) uninitialized_fill_n	destroy (C++17) destroy_n (C++17) destroy_at (C++17) construct_at (C++20)       uninitialized_default_construct (C++17) uninitialized_value_construct (C++17) uninitialized_default_construct_n (C++17) uninitialized_value_construct_n (C++17)

 

Defined in header <algorithm>
template<class ForwardIt > constexpr ForwardIt shift_left( ForwardIt first, ForwardIt last,                                 typenamestd::iterator_traits<ForwardIt>::                                     difference_type n );	(1)	(since C++20)
template<class ExecutionPolicy, class ForwardIt > ForwardIt shift_left( ExecutionPolicy&& policy,                       ForwardIt first, ForwardIt last,                       typenamestd::iterator_traits<ForwardIt>::                           difference_type n );	(2)	(since C++20)
template<class ForwardIt > constexpr ForwardIt shift_right( ForwardIt first, ForwardIt last,                                  typenamestd::iterator_traits<ForwardIt>::                                      difference_type n );	(3)	(since C++20)
template<class ExecutionPolicy, class ForwardIt > ForwardIt shift_right( ExecutionPolicy&& policy,                        ForwardIt first, ForwardIt last,                        typenamestd::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 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, 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 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(constauto& s : v){ifconstexpr(std::is_same_v<T, S>) s.specified_state? os << s.value<<' ': os <<". ";elseifconstexpr(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);// dittostd::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 (C++11)	moves a range of elements to a new location (function template)
move_backward (C++11)	moves a range of elements to a new location in backwards order (function template)
rotate	rotates the order of elements in a range (function template)
ranges::shift_leftranges::shift_right (C++23)	shifts elements in a range (algorithm function object)