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; \

What if the sharedInstance is destroyed anywhere? Can it be created again?

No, it acts like a traditional singleton in that respect - only one can ever be created.

This code is error in Xcode 4.6

GCDSingleton.h:7:25: note: expanded from macro 'DEFINE_SHARED_INSTANCE_USING_BLOCK'
  return _sharedObject; \

Last \ is not need.

Thank you for sharing such a great technique! Here is a slightly modified version:

#define SHARED_INSTANCE(...) ({\
    static dispatch_once_t pred;\
    static id sharedObject;\
    dispatch_once(&pred, ^{\
        sharedObject = (__VA_ARGS__);\
    });\
    sharedObject;\
})

It allows two forms of shared object initialization: one-line

+ (instancetype) sharedInstance {
    return SHARED_INSTANCE( [[self alloc] init] );
}

// after pre-processing:

+ (instancetype) sharedInstance {
    return ({
        static dispatch_once_t pred;
        static id sharedObject;
        dispatch_once(&pred, ^{
            sharedObject = ( [[self alloc] init] );
        });
        sharedObject;
    });
}

and multi-line (notice curly braces around the block of code):

+ (instancetype) sharedInstance {
    return SHARED_INSTANCE({
        NSLog(@"creating shared instance");
        CGFloat someValue = 84 / 2.0f;
        [[self alloc] initWithSomeValue:someValue]; // no return statement
    });
}

// after pre-processing:

+ (instancetype) sharedInstance {
    return ({
        static dispatch_once_t pred;
        static id sharedObject;
        dispatch_once(&pred, ^{
            sharedObject = ({
                NSLog(@"creating shared instance");
                CGFloat someValue = 84 / 2.0f;
                [[self alloc] initWithSomeValue:someValue];
            });
        });
        sharedObject;
    });
}

It can be used in the right part of an assignment as well:

- (void) someMethod {
    MethodPrivateHelper *helper = SHARED_INSTANCE( [[MethodPrivateHelper alloc] init] );
    // do smth with the helper
}

This modification utilizes two language features: GCC compound expressions extension, which is also supported by Clang, and C99 variadic macros support.

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