while you can indeed swap in-place for PODs [1], this is not true for c++ objects. Also, in-place swapping is not sufficient for in-place sorting.

[1] example: int a, b; a ^= b ^= a ^= b;

technically quick sort needs O(log N) additional space and it is still considered in-place. I guess the threshold for in-place-ness would be less than linear additional space?

But "swap space" without the "in" is pretty ambiguous. If it just holds 1 or 4 elements as you perform a single logical swap, you're still sorting in-place.

(This sort does not use the term that way, but another sort could.)