Re: How to implement readonly property
Re: How to implement readonly property
- Subject: Re: How to implement readonly property
- From: Ken Thomases <email@hidden>
- Date: Fri, 07 Dec 2012 20:57:40 -0600
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
_______________________________________________
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