This project contains two different implementations of Objective-C class clusters.
NSNumber is a class cluster with one public base class and many private
subclasses, one for each initializer.
A naive solution for this is to simply
have the init methods return the appropriate subclass. This has a small cost
associated as each time you call [NSNumber alloc] you would create an object
and then destroy it because initWithInt: releases the new instance and returns
a new one. A better approach is to create a singleton NSNumber instance and
return it every time you call [NSNumber alloc].
Going a step further you can
return a distinct subclass private subclass on [NSNumber alloc] and have it
manage all your cluster initializers while NSNumber manages methods common to
all subclasses. This approach can be seen in the runtime headers for
NSNumber
and NSPlaceholderNumber.
InitializerLevelCluster demonstrates this approach by returning a singleton
PlaceholderInitializerLevelCluster on alloc and a child from there.
Classes like NSString are also clusters but return different subclasses based
on how they're allocated (among other things). @"hello", [[NSString alloc] initWithFormat:@"Hello, %@", friend] and [[NSMutableString alloc] initWithString:@"mutate me!"] can all return distinct subclasses of NSString
if they so choose.
AllocLevelCluster demonstrates this by returning an instance of
ConcreteAllocLevelCluster on [AllocLevelCluster alloc] and
MutableAllocLevelCluster on [MutableAllocLevelCluster alloc].
Reference counting rules for alloc/init are pretty straight-forward.
Whatever leaves alloc needs to have a +1 reference count. If you
call [super alloc] or any other classe's alloc, it sees the +1 from the
function call and doesn't do an extra retain. If you return a singleton it
knows that it's a +0 and will do a retain to get it to +1.
Whatever enters init gets a -1 and whatever leaves init gets a +1, so if you
return something other than the original self, self gets released cancelling
the retain from the alloc. Similarly, ARC will see that you alloc/init'd
what you're returning and not do an extra retain.