iphone - 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 mutablePerson: person {
  NSMutableArray *friendArray;
}

I just tried that exact code, and Xcode gave me a compile error. I'm wondering if there is a way around it.

The project I am working on is going to have a lot of this sort of situation. I understand that I can use casts to make the code work. But I will be making an awful lot of casts if I do that, and I'm wondering if there is a better way.

Answer 1

No, you can't redeclare ivars at all. However, you can make a new method based property without making a new ivar.

@property (nonatomic, copy) NSMutableArray* mutableFriends;

@implementation MutablePerson

- (NSMutableArray*)mutableFriends {
  return (NSMutableArray*)friendArray;
}

- (void)setMutableFriends:(NSMutableArray*)friends {
  self.friendsArray = [friends mutableCopy];
}

@end