无法初始化类对象-XCode

Question

执行文件如下：

   @implementation Utils    
   +(id)alloc
   {
         return [self instance];
   }
   +(Utils *)instance
   {
        static Utils *utils = nil;
        if (!utils) {
           utils = [self init];
        }
        return utils;
   }
  -(Utils *)init
  {
       self = [super init];
       if (self) {
           mConst = [Constants instance];
           mCONT_REGEXP = [mConst CONT_REGEXP];
       }
       return self;
   }

在调用[Utils instance];时报错：

 *** Terminating app due to uncaught exception 'NSInvalidArgumentException', reason: '*** +[Utils<0xbff54> init]: cannot init a class object.'

Answer 1

+ (id)sharedInstance;
{
    static dispatch_once_t onceToken;
    static Utils *sharedUtilsInstance = nil;

    dispatch_once( &onceToken, ^{
        sharedUtilsInstance = [ [Utils alloc] init];
    });

    return sharedUtilsInstance;
}