Macro for creating your "shared instance" using GCD

ExampleClass.m

@implementation MySharedThing

// Using method 1
//
DEFINE_SHARED_INSTANCE_FOR_CLASS(MySharedThing)

// Using method 2
//
+ (id)sharedInstance
{
  DEFINE_SHARED_INSTANCE_USING_BLOCK(^{
    return [[self alloc] init];
  });
}

@end

GCDSingleton.h

//
//  GCDSingleton.h
//
//  Macros to simplify creating of a sharedInstance for a class using GCD (ARC compliant)
//
//  Use one of the following methods to create a sharedInstance
//

//
//  METHOD #1: use this if you want the sharedInstance method to be 'sharedClassName'
//
//  @implementation MySharedThing
//
//  DEFINE_SHARED_INSTANCE_FOR_CLASS(MySharedThing)
//

#define DEFINE_SHARED_INSTANCE_FOR_CLASS(className) \
+ (id)shared##className \
{ \
static dispatch_once_t pred = 0; \
__strong static id _sharedObject = nil; \
dispatch_once(&pred, ^{ \
_sharedObject = ^{return [[self alloc] init];}(); \
}); \
return _sharedObject; \
} \


//
// METHOD #2: use this if you want to name the 'sharedXXX' method yourself
//
//  @implementation MySharedThing
//
//  + (id)sharedInstance
//  {
//      DEFINE_SHARED_INSTANCE_USING_BLOCK(^{
//          return [[self alloc] init];
//      });
//  }
//
//  @end
//

#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; \

Thanks to @lukeredpath for the original.