Objective C — narrow instance variable types in subclasses?

Question

Is it possible to narrow the allowed type of an ivar in a subclass. Something like this:

@interface person: NSObject {
  NSArray *friendArray;
}

@interface         


        

Answer 1

If the compiler says 'no' then that's your answer. I would use accessors:

//Person
-(NSArray *)friends;


//MutablePerson
-(void)addFriend:(person *)friend;

In the MutablePerson you could declare another array to store friends or you could access the ivar directly in addFriend::

-(void)addFriend:(person *)friend
{
    _friendsArray = [_friendsArray arrayByAddingObject: friend];
}    

Accessing ivars directly isn't smart. I would reconsider your class design. What's the rationale for having a mutable and immutable versions of person?