Re: [boost] [Atomic] Rationale for preventing copyconstruction/assignment?

On Thursday 30 May 2013 23:05:10 Rob Stewart wrote:

On May 30, 2013, at 5:34 AM, "Peter Dimov" wrote:

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Rob Stewart wrote:

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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). While what you say is true, the two are different from a sequential consistency POV. Since the default for atomics is sequential consistency, such behavior would not be correct.

Frankly, I don't quite understand this. The standard defines sequential consistency with regard to a single particular atomic value. I.e. both "a1 = a2" and "r1 = a2" must observe the effects of the last modification of a2 that happened before the operation. If you imply that some another side effect can happen in between "r1 = a2" and "a1 = r1" then that effect doesn't qualify that definition and need not be observed.