Thread safe instantiation of a singleton

Question

Which one synchronization method to use to ensure a singleton remains a singleton?

+(Foo*)sharedInstance
{
   @synchronized(self)
   {
      if (nil == _shar         


        

Answer 1

If anyone cares, here is a Macro for the same thing:

   /*!
    * @function Singleton GCD Macro
    */
    #ifndef SINGLETON_GCD
    #define SINGLETON_GCD(classname)                            \
                                                                \
    + (classname *)shared##classname {                          \
                                                                \
        static dispatch_once_t pred;                            \
        static classname * shared##classname = nil;             \
        dispatch_once( &pred, ^{                                \
            shared##classname = [[self alloc] init];            \
        });                                                     \
        return shared##classname;                               \
    }                                                           
    #endif