On Wed, Jan 25, 2012 at 9:35 AM, Steven Schveighoffer <schvei...@yahoo.com> wrote: > On Wed, 25 Jan 2012 09:50:57 -0500, Mars <-@-.-> wrote: > >> Alternative approach, I just found: >> http://pastie.org/private/1jlcvfostnbopfp3quflg >> If I get that right, this is basically the classic singleton, like it >> would be in other languages, right? >> So... what's the best way? > > > This is an ok approach, but you must handle all threading issues manually. > In fact, you need to even with the shared version. I don't know how > threading support works with MySQL, so it may be ok just to ignore threading > issues. I'm not sure. > > Another approach is to use thread local storage to have a singleton per > instance. This avoids the whole problem of sharing the instance. > > An issue with your singleton allocation, is that you don't do the check for > the instance being null while synchronized. The singleton pattern looks > like this: > > static T instance; > > T get() > { > if(instance is null) > { > synchronized if(instance is null) > { > instance = new T; > } > } > return instance; > } > > The second check is necessary to avoid allocating multiple instances (they > will get thrown away, but no need to create them). > > Normally, you'd mark instance as volatile, but D2 doesn't support volatile > any more. I don't know the correct way to make sure the second check isn't > optimized out.
In the language spec, shared is supposed to guarantee this, but it currently doesn't. There was a conversation where pretty much every safe variant of the singleton pattern was discussed recently: http://www.digitalmars.com/d/archives/digitalmars/D/learn/Singleton_Pattern_31406.html
