这不应该是线程安全的，避免第一次调用后昂贵的锁定吗?

+ (MySingleton*)sharedInstance
{
    if (sharedInstance == nil) {
        @synchronized(self) {
            if (sharedInstance == nil) {
                sharedInstance = [[MySingleton alloc] init];
            }
        }
    }
    return (MySingleton *)sharedInstance;
}

这也适用于非垃圾收集环境。

@interface MySingleton : NSObject {
}

+(MySingleton *)sharedManager;

@end


@implementation MySingleton

static MySingleton *sharedMySingleton = nil;

+(MySingleton*)sharedManager {
    @synchronized(self) {
        if (sharedMySingleton == nil) {
            [[self alloc] init]; // assignment not done here
        }
    }
    return sharedMySingleton;
}


+(id)allocWithZone:(NSZone *)zone {
    @synchronized(self) {
        if (sharedMySingleton == nil) {
            sharedMySingleton = [super allocWithZone:zone];
            return sharedMySingleton;  // assignment and return on first allocation
        }
    }
    return nil; //on subsequent allocation attempts return nil
}


-(void)dealloc {
    [super dealloc];
}

-(id)copyWithZone:(NSZone *)zone {
    return self;
}


-(id)retain {
    return self;
}


-(unsigned)retainCount {
    return UINT_MAX;  //denotes an object that cannot be release
}


-(void)release {
    //do nothing    
}


-(id)autorelease {
    return self;    
}


-(id)init {
    self = [super init];
    sharedMySingleton = self;

    //initialize here

    return self;
}

@end

我没有读完所有的解，所以如果这段代码是多余的，请原谅。

在我看来，这是最线程安全的实现。

+(SingletonObject *) sharedManager
{
    static SingletonObject * sharedResourcesObj = nil;

    @synchronized(self)
    {
        if (!sharedResourcesObj)
        {
            sharedResourcesObj = [[SingletonObject alloc] init];
        }
    }

    return sharedResourcesObj;
}

关于Objective-C中单例模式的深入讨论，请看这里:

在Objective-C中使用单例模式

如何

static MyClass *gInstance = NULL;

+ (MyClass *)instance
{
    if (gInstance == NULL) {
        @synchronized(self)
        {
            if (gInstance == NULL)
                gInstance = [[self alloc] init];
        }
    }

    return(gInstance);
}

这样就避免了初始化后的同步成本?

关于单例宏代码的详细解释在Cocoa With Love博客上

http://cocoawithlove.com/2008/11/singletons-appdelegates-and-top-level.html。