std::rotate_copy() algorithm

od C++20

// (1)
template< class ForwardIt, class OutputIt >
constexpr OutputIt rotate_copy( ForwardIt first, ForwardIt n_first,
                                ForwardIt last, OutputIt d_first );

// (2)
template< class ExecutionPolicy, class ForwardIt1, class ForwardIt2 >
ForwardIt2 rotate_copy( ExecutionPolicy&& policy,
                        ForwardIt1 first, ForwardIt1 n_first,
                        ForwardIt1 last, ForwardIt2 d_first );

od C++17

// (1)
template< class ForwardIt, class OutputIt >
OutputIt rotate_copy( ForwardIt first, ForwardIt n_first,
                      ForwardIt last, OutputIt d_first );

// (2)
template< class ExecutionPolicy, class ForwardIt1, class ForwardIt2 >
ForwardIt2 rotate_copy( ExecutionPolicy&& policy,
                        ForwardIt1 first, ForwardIt1 n_first,
                        ForwardIt1 last, ForwardIt2 d_first );

do C++17

// (1)
template< class ForwardIt, class OutputIt >
OutputIt rotate_copy( ForwardIt first, ForwardIt n_first,
                      ForwardIt last, OutputIt d_first );

• (1) Copies the elements from the range [first; last), to another range beginning at d_first in such a way, that the element *(n_first) becomes the first element of the new range and *(n_first - 1) becomes the last element.

• (2) Same as (1), but executed according to policy.

  Overload Resolution

  These overloads participate in overload resolution only if std::is_execution_policy_v<std::decay_t<ExecutionPolicy>>  (do C++20) std::is_execution_policy_v<std::remove_cvref_t<ExecutionPolicy>>  (od C++20) is true.

Undefined Behaviour

The behavior is undefined if either [first; n_first) or [n_first; last) is not a valid range, or the source and destination ranges overlap.

Parameters

first last	The range of elements to copy.
n_first	An iterator to an element in [first; last) that should appear at the beginning of the new range.
d_first	The beginning of the destination range.
policy	The execution policy to use. See execution policy for details.

Type requirements

ForwardIt1 ForwardIt2 ForwardIt3	LegacyForwardIterator
OutputIt	LegacyOutputIterator

Return value

Output iterator to the element past the last element copied.

Complexity

Given N as std::distance(first, last):

Linear in N.

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.

Possible implementation

rotate_copy (1)

template<class ForwardIt, class OutputIt>
constexpr // since C++20
OutputIt rotate_copy(ForwardIt first, ForwardIt n_first,
                     ForwardIt last, OutputIt d_first)
{
    d_first = std::copy(n_first, last, d_first);
    return std::copy(first, n_first, d_first);
}

Examples

The following copy-prints a vector, replacing all values over 5 with 99 on the fly.

Main.cpp

#include <algorithm>
#include <iostream>
#include <iterator>
#include <vector>

int main()
{
    std::vector<int> src {1, 2, 3, 4, 5};
    std::vector<int> dest(src.size());
    auto pivot = std::find(src.begin(), src.end(), 3);

    std::rotate_copy(src.begin(), pivot, src.end(), dest.begin());
    for (int i : dest)
        std::cout << i << ' ';
    std::cout << '\n';

    // copy the rotation result directly to the std::cout
    pivot = std::find(dest.begin(), dest.end(), 1);
    std::rotate_copy(dest.begin(), pivot, dest.end(),
                     std::ostream_iterator<int>(std::cout, " "));
    std::cout << '\n';
}

Output

3 4 5 1 2
1 2 3 4 5