由于Kendall发布了一个线程安全的单例，试图避免锁定成本，我想我也会抛出一个:

#import <libkern/OSAtomic.h>

static void * volatile sharedInstance = nil;                                                

+ (className *) sharedInstance {                                                                    
  while (!sharedInstance) {                                                                          
    className *temp = [[self alloc] init];                                                                 
    if(!OSAtomicCompareAndSwapPtrBarrier(0x0, temp, &sharedInstance)) {
      [temp release];                                                                                   
    }                                                                                                    
  }                                                                                                        
  return sharedInstance;                                                                        
}

好吧，让我来解释一下这是怎么回事:

Fast case: In normal execution sharedInstance has already been set, so the while loop is never executed and the function returns after simply testing for the variable's existence; Slow case: If sharedInstance doesn't exist, then an instance is allocated and copied into it using a Compare And Swap ('CAS'); Contended case: If two threads both attempt to call sharedInstance at the same time AND sharedInstance doesn't exist at the same time then they will both initialize new instances of the singleton and attempt to CAS it into position. Whichever one wins the CAS returns immediately, whichever one loses releases the instance it just allocated and returns the (now set) sharedInstance. The single OSAtomicCompareAndSwapPtrBarrier acts as both a write barrier for the setting thread and a read barrier from the testing thread.

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。

根据我下面的另一个回答，我认为你应该这样做:

+ (id)sharedFoo
{
    static dispatch_once_t once;
    static MyFoo *sharedFoo;
    dispatch_once(&once, ^ { sharedFoo = [[self alloc] init]; });
    return sharedFoo;
}

KLSingleton是: 可子类化(到n阶) 弧相容 使用alloc和init是安全的 装载 线程安全的 无锁(使用+initialize，而不是@synchronize) Macro-free Swizzle-free 简单的

克林格尔顿

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;
}

static mySingleton *obj=nil;

@implementation mySingleton

-(id) init {
    if(obj != nil){     
        [self release];
        return obj;
    } else if(self = [super init]) {
        obj = self;
    }   
    return obj;
}

+(mySingleton*) getSharedInstance {
    @synchronized(self){
        if(obj == nil) {
            obj = [[mySingleton alloc] init];
        }
    }
    return obj;
}

- (id)retain {
    return self;
}

- (id)copy {
    return self;
}

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

- (void)release {
    if(obj != self){
        [super release];
    }
    //do nothing
}

- (id)autorelease {
    return self;
}

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