Understanding the scope of child/parent instance variables

Question

I have a question regarding variable scope with subclasses.

I have something like the following:

ParentClass.h

@interface ParentClass : NSObject

@property (nonatomic, strong, readonly) NSString *myIdentifier;

- (id)initWithIdentifier:(NSString *)identifier;

@end

ParentClass.m

@interface ParentClass() 
@property (nonatomic, strong) NSString *myIdentifier;
@end

@implementation ParentClass 

- (id)initWithIdentifier:(NSString *)identifier {
    if (self = [super init]) {
       self.myIdentifier = identifier;
    }
    return self;
}
@end

ChildClass.h

@interface ChildClass : ParentClass

- (NSString *)showMeTheIdentifier;

@end

ChildClass.m

@implementation ChildClass

- (NSString *)showMeTheIdentifier {
    return self.myIdentifier;
}

Then I go to implement the class and I get no compiler warnings, but the result of the NSLog is (null)

ViewController.m

...
ChildClass *c = [[ChildClass alloc] initWithIdentifier:@"abc123"];
NSLog(@"What's my identifier? %@", [c showMeTheIdentifier]);
...

What am I doing wrong?

Thanks

Are you missing the `self = [super init]` stuff in your `ParentClass` initializer, or did you leave it out for brevity in the question? — veddermatic, Mar 13 '14 at 12:38
move `@property (nonatomic, strong) NSString *myIdentifier;` to .h — Anoop Vaidya, Mar 13 '14 at 12:39
sorry... yeah... have the `self = [super init]` part - just forgot it in the pseudo-code — Joseph, Mar 13 '14 at 12:41
Looks fine to me. It should work. What is it logging exactly? — Shanti K, Mar 13 '14 at 12:43
@Anoop I don't want the identifier to be changed from the *outside*. That is why it is a `readonly`. The only way to change it should be from the `init` method. — Joseph, Mar 13 '14 at 12:44
i have created a dummy proj with exactly whatever u have typed in here.. It works for me.. it logs What's my identifier? abc123 Are you sure you are not missing anything? — Shanti K, Mar 13 '14 at 12:48
I checked your code, its perfectly working in my system. Do a **clean+build** and check again. — Anoop Vaidya, Mar 13 '14 at 12:48
Remove the property in the private category and use this in the init. _myIdentifier = identifier; — Shanti K, Mar 13 '14 at 12:50
removed the `readonly` in the public, edited to the `init` to use `_myidentifier` and commented out the `@property...` declaration in the .m file. Still no luck... very strange... — Joseph, Mar 13 '14 at 12:56

Shanti K · Answer 1 · 2014-03-13T12:50:02.737

1

You can remove the property declaration in the private category and do something like this:

You are not returning self in init. Hence it does not work.

- (id)initWithIdentifier:(NSString *)identifier {
    self = [super init];
    if(self) {
        _myIdentifier = identifier;
    }
    return self;
}

edited Mar 13 '14 at 12:50

answered Mar 13 '14 at 12:40

Shanti K

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like I commented - that's not the problem. Just forgot to type it in my pseudo-code – Joseph Mar 13 '14 at 12:42

score 1 · Accepted Answer · answered Mar 13 '14 at 12:57

Aside from the fact that you have self == [super init] instead of self = [super init] in your "pseudo code" (you're comparing self to [super init] instead of assigning it), everything looks fine. If you're are using == in your actual code, self is returning nil. You may want to try logging the actual child class and see if the object exists after you instantiate it (or set a break point and check the debugger). That'll tell you if the problem is in instantiation or elsewhere.

Most likely, there's some other detail in your actual code that's causing the problem. It's possible that something you think isn't related is actually the problem. I've done this before when posting questions. I try to cut out the "cruft" and end up also cutting out the problem. You may want to try posting your actual code.

you are right - I could kick myself. I had an `if-statment` that was preventing the equivalent statement to my pseudo `self.myIdentifier = identifier` :head:desk: — Joseph, Mar 13 '14 at 13:01

Understanding the scope of child/parent instance variables

2 Answers2