Macro for creating your "shared instance" using GCD

ExampleClass.m

@implementation MySharedThing

+ (id)sharedInstance
{
  DEFINE_SHARED_INSTANCE_USING_BLOCK(^{
    return [[self alloc] init];
  });
}

@end

GCDSingleton.h

#define DEFINE_SHARED_INSTANCE_USING_BLOCK(block) \
  static dispatch_once_t pred = 0; \
  __strong static id _sharedObject = nil; \
  dispatch_once(&pred, ^{ \
    _sharedObject = block(); \
  }); \
  return _sharedObject; \

I am afraid that in this way it can't prevent creating multiple instances by 'alloc'.