Cocoa Singleton conventions

Question

Cocoa is full of singletons. Is there a logical/conventional difference between when the Cocoa APIs use

NSSingletonObject *so = [NSSingletonObject defaultSingleton];

versus

NSSingletonObject *so = [NSSingletonObject sharedSingleton];

?

Not a huge thing, but I don't really see why sometimes one is used versus the other.

Answer 1

I think it tends to be that if it is a true singleton (such as NSApplication) that you are using, then the -[JKFoo sharedFoo] convention is followed. If on the other hand the class provides access to a default instance but you can still create other instances (for example NSNotificationQueue or NSFileManager) then the -[JKBar defaultBar] convention is used.

Side note: if you are implementing a few of your own Cocoa singletons, then there is a useful OpenSource header you might want to take a look at :)