Objective-c singleton

Singleton.h

//from http://boredzo.org/blog/archives/2009-06-17/doing-it-wrong
@interface SingletonManager : NSObject {}

+ (id)sharedManager;

@end

Singleton.m

static SingletonManager *sharedInstance = nil;

@implementation SingletonManager

#pragma mark -
#pragma mark Singleton

+ (void)initialize{
    if(!sharedInstance){
        [[[self alloc] init] release];
    }
}

+ (id)sharedManager{
    return sharedInstance;
}

+ (id)allocWithZone:(NSZone *)zone{
    return [sharedInstance retain] ?: [super allocWithZone:zone];
}

- (id)init{
    if(!sharedInstance){
        if ((self = [super init])){
            //initialize ivars
        }
        sharedInstance = [self retain];
    }else if(self != sharedInstance){
        [self release];
        self = [sharedInstance retain];
    }
    
    return self;
}

@end