It has runtime overhead whenever a move assignment occurs. You have to check if the target of the assignment is null (or otherwise call the destructor) and you

It has runtime overhead whenever a move assignment occurs. You have to check if the target of the assignment is null (or otherwise call the destructor) and you have to assign null to the source. This is all overhead compared to a plain pointer assignment. An optimizing compiler will of course try to eliminate most of that overhead, but that's not always possible.