-----Original Message----- From: boost-bounces@lists.boost.org [mailto:boost-bounces@lists.boost.org] On Behalf Of Peter Dimov Sent: Monday, February 14, 2005 2:38 PM To: boost@lists.boost.org Subject: Re: [boost] singleton usage
A singleton is simply a glorified wrapper over several global variables and functions. There is little difference between
Singleton::instance().f();
and
Singleton::f();
where the second Singleton is a namespace. The global variables don't become any less "evil" when they are made members of a singleton class.
When a class represents an out-of-program entity that, by its nature, has a single instance, this class can be flattened into functions.
This isn't black and white, of course; today's code may treat the video card as a singleton, but tomorrow's computers may add multiple video card support. In this case I still prefer something along the lines of
VideoCard & get_video_card( int id ); // noncopyable
or maybe even
video_card_ref get_video_card( int id ); // reference copy semantics
instead of the "multisingleton" way of
VideoCard::instance( id ).
But "true" singletons are either crippled ordinary classes or collections of global functions (under the assumption that there are no initialization order issues, of course).
Thank you for your comments. I agree with everything you said and try to avoid singletons in the first place. However, I have a practical problem that I have been able to "solve" only thru singletons. In a header only library that uses a class factory, the registration of objects with the factory is performed in a constructor of an object inside an anonymous namespace. Since the header may be #included more that once, I made those objects singletons to avoid multiple registration. The singleton::instance method may be called more that once without need, that's why I put "solve" in quotes. Since the object is inside an anonymous namespace, I can not use templates, which never get instantiated. If you or anyone else have a suggestion that allows for automatic registration without using singletons please share it :-) I would like to keep the anonymous namespace, but if there is a nice solution that requires a named one then I would adopt it. Thanks in advance. Best regards Jorge