The true Objective-C singleton

gistfile1.txt

+ (id)alloc {
    static dispatch_once_t onceToken;
    static SettingsViewController *singleton = nil;
    dispatch_once(&onceToken, ^{
        singleton = [super alloc];
    });
    return singleton;
}

- (id)init {
    static dispatch_once_t onceToken;
    static SettingsViewController *singleton = nil;
    dispatch_once(&onceToken, ^{
        singleton = [super init];
    });
    return singleton;
}