◐ Shell
clean mode source ↗

std::shift_left, std::shift_right - cppreference.com

From cppreference.com

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:

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

#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 & algorithm function object)[edit]
moves a range of elements to a new location in backwards order
(function template & algorithm function object)[edit]
rotates the order of elements in a range
(function template & algorithm function object)[edit]
shifts elements in a range
(algorithm function object)[edit]