An implementation of a Singleton accessor routine. Provides a dispatch_once-based version and a plain version for pre-(10.6|4.0) systems, with load-time method swapping to install the dispatch version if libdispatch is available at runtime.

SingletonInitializer.m

#import <libkern/OSAtomic.h>
#import <dispatch/dispatch.h>
#import <objc/runtime.h>

@implementation MySingleton

+ (void) load
{
	// check for weak-linked libdispatch symbols
	if ( dispatch_queue_create != 0 )
	{
		Method plainMethod = class_getInstanceMethod( self, @selector(sharedInstance) );
		Method dispatchMethod = class_getInstanceMethod( self, @selector(sharedInstance_dispatch_once) );
		method_exchangeImplementations( plainMethod, dispatchMethod );
	}
}

+ (MySingleton *) sharedInstance
{
	static MySingleton * volatile __singleton = nil;
	
	if ( __singleton == nil )
	{
		MySingleton * obj = [[self alloc] init];
		if ( OSAtomicCompareAndSwapPtrBarrier(nil, obj, (void * volatile *)&__singleton) == false )
			[obj release];	// already allocated/assigned by another thread
	}
	
	return ( __singleton );
}

+ (MySingleton *) sharedInstance_dispatch_once
{
	// let libdispatch do the work for us
	static MySingleton * volatile __singleton = nil;
	static dispatch_once_t __once = 0;
	
	dispatch_once( &__once, ^{ __singleton = [[self alloc] init]; });
	return ( __singleton );
}

@end