Question

Example: The -save: method of NSManagedObjectContext is declared like this:

- (BOOL)save:(NSError **)error

Since NSError is already a class, and passing a pointer would actually have the effect of modifying this object inside the implementation of -save:, what’s the point of passing a pointer to a pointer here? What’s the advantage/sense?

Usage example:

NSError *error;
if (![managedObjectContext save:&error]) {
    // Handle the error.
}

Answer 1

If you just passed in a pointer, all the method could do would alter the already existing NSError object that you are pointing to.

By passing in a pointer to a pointer, it can create new NSError objects and leave you with a pointer that points to them.