That still leaves the problem of when to use std::swap vs ordinary assignment in generic (i.e. templated) code.

Like when std::vector needs to resize its underlying storage (as a result of push_back, for example), it has to decide which approach to use to copy/move items from the old storage to the new storage.

For std::vector<std::string>, std::swap would probably be at least ok if not optimal, but for std::vector<int> it would be overkill and therefore decidedly non-optimal. In the latter case, you want to do memcpy(new, old) and be done, not std::swap(old[i], new[i]) for each int.

I think a lot of the motive for adding move semantics to c++ has to do with giving the compiler enough information to produce results that are both optimal and correct in generic code.