Question

I’m implementing a singleton class (and lets not get into the discussion if that is wrong or not). I have a method to get the instance of this class, which is lazily initialized:

+ (FFDataManager *)sharedDataManager {
  static FFDataManager *dm = nil;
  if (!dm) {
    dm = [[FFDataManager alloc] init];
  }

  return dm;
}

Is there anything I should look out for when doing this using static (inside of the method) as opposed to creating a global variable? Is there anything that can go wrong, all tutorials on the Internet use a global variable.

Answer 1

My preferred singleton implementation looks like:

+ (MyClass *) sharedInstance {
    static dispatch_once_t predicate = 0;
    __strong static MyClass *shared = nil;

    dispatch_once(&predicate, ^{
        shared = [[self alloc] init];
    });

    return shared;
}

Using dispatch_once makes sure this is also thread safe. Your code would allocate twice when accessed by multiple threads at the same time.