Rob Stewart wrote:
Assignment is a load and a store, as you note, so it isn't atomic, which is the point of an atomic type, after all. Between the load and store, the source atomic's value can change, so the copy can be wrong, hence Steven's question.
I don't think that the copy can be wrong, although I may be missing something. From a cursory inspection, it seems to me that a1 = a2; and r1 = a2; a1 = r1; are equivalent. It's true that in the second case a2's value can change after the first line, but it can change in the first case after the only line as well (which corresponds to changing after the second line in the second case), and the observable effect is the same (except in the trivial case in which a1 and a2 are the same variable, but that's easily taken care of).