Phil, thanks for your positive review. I almost overlooked it. Phil Endecott wrote:
Today starts the formal Fast-Track review of the Boost.Utility/Singleton library.
I've never made much use of singletons, but I won't let that stop me from commenting...
My thoughts:
1. There's no "un-synchronised" version, for use in single-threaded programs or when all (first) accesses come from the same thread.
If there is just a single thread all Singletons behave the same. If you want to avoid the initialization check within one thread you can 'lease' a non-mutexed Singleton. Yes, the creation will be synchronized but it's very unlikely that this part will become a performance bottleneck.
2. The "mutexed access" feature can be useful in situation other than singletons; can't something more general-purpose be provided? I use a template class Lockable<T> which grants reader/writer-exclusive access to a contained T. (Reader/writer locking could be useful here.) (It might be useful to make the mutex type a template parameter, with a sensible default; I have a few mutexes of my own, with a boost::mutex-compatible interface.)
You can still use 'singleton<T>' and implement a custom synchronization scheme. I think that's best if you want to use r/w locks effectively. Further, a mutex can be more efficient than r/w locks; AFAIK those only pay off if there's enough concurrency going on (I might be wrong).
3. Function static variables can be thought of as "local singletons", but they aren't synchronised. It would be great if we could replace "static T t(.....);" with "safe_static<T> t(.....);" and get the same semantics.
Is it implementable? Regards, Tobias