If your system supports blocks (like iOS 4+) then you should just use the dispatch_once function as follows:

+ (MyClass *)sharedInstance
{
    static MyClass *sharedInstance = nil;
    static dispatch_once_t onceToken;
    dispatch_once(&onceToken, ^{
        sharedInstance = [[MyClass alloc] init];
        // Do any other initialisation stuff here
    });
    return sharedInstance;
}

This method was sourced from http://stackoverflow.com/questions/7568935/how-do-i-implement-an-objective-c-singleton-that-is-compatible-with-arc courtesy Nick Forge


=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=
=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=


See the "pre-blocks" branch for the old implementation:

A Singleton Macro that swaps method implementations at runtime so that only the initialization portion will require syncrhonization (lock).
- Specifically it uses method_setImplementation() to dynamically replace the sharedInstance access method with one that does not instantiate a new object and thus does not require @synchronized.

It is based on the work here: http://cocoawithlove.com/2008/11/singletons-appdelegates-and-top-level.html  by Matt Gallagher
But changing the implementation to use method swizzling as described here: http://googlemac.blogspot.com/2006/11/synchronized-swimming-part-2.html by Dave MacLachlan of Google.