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

Subject: Re: [boost] [Atomic] Rationale for preventing copyconstruction/assignment?
From: Peter Dimov (lists_at_[hidden])
Date: 2013-05-30 05:34:14

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).