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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