公认的答案，虽然它编译，是不正确的。

+ (MySingleton*)sharedInstance
{
    @synchronized(self)  <-------- self does not exist at class scope
    {
        if (sharedInstance == nil)
            sharedInstance = [[MySingleton alloc] init];
    }
    return sharedInstance;
}

根据苹果文档:

．.． 您可以采用类似的方法来同步相关类的类方法，使用class对象而不是self。

即使使用self工作，它不应该，这看起来像一个复制和粘贴错误对我来说。 类工厂方法的正确实现应该是:

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

@interface MySingleton : NSObject
{
}

+ (MySingleton *)sharedSingleton;
@end

@implementation MySingleton

+ (MySingleton *)sharedSingleton
{
  static MySingleton *sharedSingleton;

  @synchronized(self)
  {
    if (!sharedSingleton)
      sharedSingleton = [[MySingleton alloc] init];

    return sharedSingleton;
  }
}

@end

(来源)

static MyClass *sharedInst = nil;

+ (id)sharedInstance
{
    @synchronize( self ) {
        if ( sharedInst == nil ) {
            /* sharedInst set up in init */
            [[self alloc] init];
        }
    }
    return sharedInst;
}

- (id)init
{
    if ( sharedInst != nil ) {
        [NSException raise:NSInternalInconsistencyException
            format:@"[%@ %@] cannot be called; use +[%@ %@] instead"],
            NSStringFromClass([self class]), NSStringFromSelector(_cmd), 
            NSStringFromClass([self class]),
            NSStringFromSelector(@selector(sharedInstance)"];
    } else if ( self = [super init] ) {
        sharedInst = self;
        /* Whatever class specific here */
    }
    return sharedInst;
}

/* These probably do nothing in
   a GC app.  Keeps singleton
   as an actual singleton in a
   non CG app
*/
- (NSUInteger)retainCount
{
    return NSUIntegerMax;
}

- (oneway void)release
{
}

- (id)retain
{
    return sharedInst;
}

- (id)autorelease
{
    return sharedInst;
}

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

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;
}

扩展一下@robbie-hanson的例子…

static MySingleton* sharedSingleton = nil;

+ (void)initialize {
    static BOOL initialized = NO;
    if (!initialized) {
        initialized = YES;
        sharedSingleton = [[self alloc] init];
    }
}

- (id)init {
    self = [super init];
    if (self) {
        // Member initialization here.
    }
    return self;
}