Re: How to implement readonly property
Re: How to implement readonly property
- Subject: Re: How to implement readonly property
- From: Kyle Sluder <email@hidden>
- Date: Sat, 08 Dec 2012 10:18:12 -0800
On Dec 8, 2012, at 10:06 AM, Steve Sisak <email@hidden> wrote:
> At 8:35 AM -0800 12/8/12, Kyle Sluder wrote:
>> On Dec 7, 2012, at 8:38 PM, Marco S Hyman <email@hidden> wrote:
>>
>>> On Dec 7, 2012, at 8:18 PM, Steve Sisak <email@hidden> wrote:
>>>
>>>> I'm interested if there are an any issued I'm missing in the Obj-C, @synchronized(self), instance variable case.
>>>
>>>
>>> Your pattern can fail if this line
>>> _someDictionary = temp;
>>> isn't atomic.
>>
>> The real issue with double-checked locking is whether the compiler promises to generate the proper memory barriers such that other threads are guaranteed to see the assignment to _someDictionary *after* the object has been constructed. C makes no such guarantee; other threads might see a non-nil value for _someDictionary before the first thread is done constructing the object.
>
> I'm fairly sure that @synchronized, being a compiler built-in, rather than a function, makes that guarantee -- specifically the the values of instance variables can change across entry and exit of an @synchronized block.
If you actually understood the problem with double-checked locking, you'd understand that the problem is with the early return that happens before the @synchronized block.
Please reread the Wikipedia article Ken linked to; the Java example exhibits *exactly* the same flaw as your Objective-C code.
>
> Further, if writes were not complete at the end of the block, the construct would be essentially useless for its intended purpose.
Again, the problem exists *before* entering the @synchronized block.
>
> In any case, removing the outer check makes the code correct regardless of compiler guarantees.
Yes, which is why your "optimization" is a classic anti-pattern.
--Kyle Sluder
_______________________________________________
Cocoa-dev mailing list (email@hidden)
Please do not post admin requests or moderator comments to the list.
Contact the moderators at cocoa-dev-admins(at)lists.apple.com
Help/Unsubscribe/Update your Subscription:
This email sent to email@hidden