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

Subject: Re: [boost] [Atomic] Rationale for preventing copyconstruction/assignment?
From: Rob Stewart (robertstewart_at_[hidden])
Date: 2013-05-30 23:05:10

On May 30, 2013, at 5:34 AM, "Peter Dimov" <lists_at_[hidden]> wrote:

> 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). 

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.

___
Rob

(Sent from my portable computation engine)