Sinlgeton object in objective C

Question

I need some clarification regarding the singleton object implementation in Objective C. I have implemented the following code for the singleton object ..

static MyClass *instance = nil;
+(MyClass *)getInstance 
{
    @synchronised(self)
    {
        if(instance == nil)
        {
            instance = [[self alloc] init];
        }
    }
    return instance;
}

-(void)dealloc
{
    [instance release];
    [super dealloc];
}

1. Does the singleton object requires @synchronised block ???

2. I have custom defined constructor in my class as follows:

-(id)initWithDefault ..

Does the following line of code creates an issue while allocating for instance

instance = [[self alloc] initWithDefault];

awaiting for your response.

Answer 1

http://stackoverflow.com/questions/145154/what-does-your-objective-c-singleton-look-like has several singleton implementations and links to discussion and code.

Answer 2

Yes you should. There is a really handy macro by Matt Gallagher that you can use to add singleton support for your class (you just add a SYNTHESIZE_SINGLETON_FOR_CLASS(<class name> inside the implementation block):

  #define SYNTHESIZE_SINGLETON_FOR_CLASS(classname) \
  \
  static classname *shared##classname = nil; \
   \
  + (classname *)shared##classname \
  { \
    @synchronized(self) \
    { \
     if (shared##classname == nil) \
     { \
      shared##classname = [[self alloc] init]; \
     } \
    } \
     \
    return shared##classname; \
  } \
   \
  + (id)allocWithZone:(NSZone *)zone \
  { \
    @synchronized(self) \
    { \
     if (shared##classname == nil) \
     { \
      shared##classname = [super allocWithZone:zone]; \
      return shared##classname; \
     } \
    } \
     \
    return nil; \
  } \
  \
  - (id)copyWithZone:(NSZone *)zone \
  { \
    return self; \
  } \
 \
  - (id)retain \
  { \
    return self; \
  } \
  \
  - (NSUInteger)retainCount \
  { \
    return NSUIntegerMax; \
  } \
  \
  - (id)autorelease \
  { \
    return self; \
  }