On Dec 7, 2012, at 8:01 PM, Steve Sisak wrote:
> Here's what I usually do:
>
> assume that _someDictionary is an instance variable initialized to nil and
> never changed once initialized to non-nil
>
> - (NSDictionary *)someDictionary;
> {
> if (!_someDictionary)
> {
> @synchronized (self)
> {
> if (!_someDictionary)
> {
> // create a temp dictionary (might take some time)
> _someDictionary = temp;
> }
> }
> }
>
> return _someDictionary;
> }
>
> the outer if avoids the overhead of @synchronized if _someDictionary is
> already created -- this is just an optimization
>
> the inner if is necessary to resolve the race condition if multiple threads
> make it past the outer one
This is a classic anti-pattern called double-checked locking. It is not safe.
Don't rely on it.
https://en.wikipedia.org/wiki/Double-checked_locking
http://erdani.com/publications/DDJ_Jul_Aug_2004_revised.pdf
Regards,
Ken
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