Re: Singletons with ARC
Re: Singletons with ARC
- Subject: Re: Singletons with ARC
- From: Ken Thomases <email@hidden>
- Date: Fri, 09 Dec 2011 00:55:00 -0600
On Dec 8, 2011, at 11:18 PM, Abdul Sowayan wrote:
> Hi Ben,
>
>> I'm rather confused by the commonly used implementation of singletons. (Ignoring the fact that perhaps I shouldn't be using singletons) I would appreciate you view on this.
>>
>> Normally I implement a singleton such as..
>>
>> static MyClass *sharedInstance = nil;
>> + (MyClass *)sharedInstance
>> {
>> @synchronized(self)
>> {
>> if (sharedInstance == nil)
>> sharedInstance = [[self alloc] init];
>> }
>> return(sharedInstance);
>> }
>
> The above will still work in an ARC environment. It is rather inefficient, however. For details please check the "Double Check Locking Optimization pattern":
> http://www.cs.wustl.edu/~schmidt/PDF/DC-Locking.pdf
Double-checked locking is broken. It is an anti-pattern in many languages, including the C family under most common implementations. Don't use it. Google it if you want to confirm. One reference: <http://www.cs.umd.edu/~pugh/java/memoryModel/DoubleCheckedLocking.html>.
Regards,
Ken
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