std::shift_left, std::shift_right

 

C++

Compiler support

Freestanding and hosted

Language

Standard library

Standard library headers

Named requirements

Feature test macros (C++20)

Language support library

Concepts library (C++20)

Metaprogramming library (C++11)

Diagnostics library

General utilities library

Strings library

Containers library

Iterators library

Ranges library (C++20)

Algorithms library

Numerics library

Localizations library

Input/output library

Filesystem library (C++17)

Regular expressions library (C++11)

Concurrency support library (C++11)

Execution support library (C++26)

Technical specifications

Symbols index

External libraries

 

Algorithm library

Constrained algorithms and algorithms on ranges (C++20)
Constrained algorithms, e.g. ranges::copy, ranges::sort, ...
Execution policies (C++17)
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)
Non-modifying sequence operations Batch operations for_each for_each_n (C++17) Search operations all_ofany_ofnone_of (C++11)                (C++11)(C++11) countcount_if mismatch equal findfind_iffind_if_not (C++11) find_end find_first_of adjacent_find search search_n Modifying sequence operations Copy operations copycopy_if (C++11) copy_backward copy_n (C++11) move (C++11) move_backward (C++11) Swap operations swap iter_swap swap_ranges Transformation operations replacereplace_if transform replace_copyreplace_copy_if Generation operations fill fill_n generate generate_n Removing operations removeremove_if unique remove_copyremove_copy_if unique_copy Order-changing operations reverse reverse_copy rotate rotate_copy random_shuffleshuffle (until C++17)(C++11) shift_leftshift_right (C++20)(C++20) Sampling operations sample (C++17)	Sorting and related operations Partitioning operations partition partition_copy (C++11)   stable_partition is_partitioned (C++11) partition_point (C++11) Sorting operations sort stable_sort partial_sort partial_sort_copy is_sorted (C++11) is_sorted_until (C++11) nth_element Binary search operations (on partitioned ranges) lower_bound upper_bound equal_range binary_search Set operations (on sorted ranges) includes set_union set_intersection set_difference set_symmetric_difference Merge operations (on sorted ranges) merge inplace_merge Heap operations push_heap pop_heap make_heap sort_heap is_heap (C++11) is_heap_until (C++11) Minimum/maximum operations max min minmax (C++11) clamp (C++17) max_element min_element minmax_element (C++11) Lexicographical comparison operations lexicographical_compare lexicographical_compare_three_way (C++20) Permutation operations next_permutation prev_permutation is_permutation (C++11) C library qsort bsearch
Numeric operations
iota (C++11) inner_product adjacent_difference    accumulate reduce (C++17) transform_reduce (C++17)	partial_sum inclusive_scan (C++17)    exclusive_scan (C++17) transform_inclusive_scan (C++17) transform_exclusive_scan (C++17)
Operations on uninitialized memory
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,                                 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 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 (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 (niebloid)