[wakawaka28]

The problem seems less arbitrary if the chunks being rotated are large enough. Implicit in the problem is that any method that would require additional memory to be allocated would probably require memory proportional to the sizes of stuff being swapped. That could be unmanageable.

As for whether std::rotate() uses allocations, I can't say without looking. But I know it could be implemented without allocations. Maybe it's optimal in practice to use extra space. I don't think a method involving reversal of items is generally going to be the fastest. It might be the only practical one in some cases or else better for other reasons.