On Mar 8, 2012, at 2:15 PM, Quincey Morris wrote:
> On Mar 8, 2012, at 12:18 , CoGe - Tamas Nagy wrote:
>
>> I should subclass NSMutableArray because I had a project which used
>> NSMutableArray (calls) before, but was not thread safe. Anyway, I can't
>> really understand why it would be _really_ better to implement just
>> accessors, but maybe you could explain it a bit more?
>
> Well, slap my head if I'm missing your intention, but I don't see how having
> an *atomically* safe NSMutableArray helps at all. As soon as anyone in any
> thread writes:
>
> for (NSInteger i = 0; i <= atomicallySafeArray.count; i++) {
> id object = [atomicallySafeArray objectAtIndex: i];
> ...
> }
>
> or any such construct that contains 2+ references to the array that might see
> a different state of the array over time, then the code is broken and not
> thread safe. This is also broken:
>
> for (id object in atomicallySafeArray)
> …
>
> for the separate but related reason that fast enumeration will fail if the
> array mutates.
>
> Also, implementing the accessors won't solve either of these problems.
>
> Thread safety does *not* come from atomicity. Conversely, in many cases
> atomicity has no value whatsoever -- because thread safety is what's really
> required.
And this is why atomic-by-default is a misfeature. Slower code that contributes
to a false impression of thread-safety.
--Kyle Sluder
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