Accessing array in parent returns no objects

Question

I must be overthinking this simple parent and subclass communication in Objective-C.

I am creating and filling an NSMutableArray in the parent View Controller, then in the subclass View Controller I want to access the objects in that array. Xcode is not throwing any errors, but in the subclass it is not finding any objects in the array. What am I doing wrong?

Parent.h:

@interface ParentViewController : UIViewController
@property (strong, nonatomic) NSMutableArray *myArray;
@end

Parent.m:

[self.myArray addObject:myString]; //add objects to array in a method

Subclass.h:

#import "ParentViewController.h"
@interface SubclassViewController : ParentViewController
@end

Subclass.m:

#import "SubclassController.h"
@implementation SubclassViewController
- (void)myMethod {
    NSLog(@"%d in parent array", [super.myArray count]); //always returns 0
}
@end

myArray is never initialised.

max_
– max_

2014-01-27 00:28:19 +00:00
Commented Jan 27, 2014 at 0:28 — max_
– max_, Commented Jan 27, 2014 at 0:28

Mischa · Accepted Answer · 2014-01-27 00:46:54Z

3

It depends on where you put this line in your code (in which method):

[self.myArray addObject:myString]; //add objects to array in a method

You can for example put it in your parentViewController's -viewDidLoad method and if you call [super viewDidLoad] in your SubclassViewController (or you do not implement the viewDidLoad method in your SubclassViewController at all) it will actually work.

The problem is that you are mixing up a class and an object: What you see in the end on screen is an instance of your class parentViewControllerand an instance of your class SubclassViewController i.e. they are two distinct objects. Just like Sam and Bob are two different "objects" of the class "Human". Now they both have a property myArray - like a list that Sam and Bob both carry. But as they are two different individuals they also carry different lists. And if Sam writes some notes on his list it does not mean that those notes automagically appear on Bob's list as well.

edited Jan 27, 2014 at 0:46

answered Jan 27, 2014 at 0:26

Mischa

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1 Comment

Jordan H Over a year ago

Thank you for the explanation. :) I now realize subclasses inherit the properties but they are different. Inheritance wasn't what I wanted then, and instead I used the prepareForSegue method to send my mutable array to the other view controller as an immutable array (it's own property) which is great, since that view controller shouldn't change it anyways.

Abhinav · Accepted Answer · 2014-01-27 00:18:11Z

2

You need to allocate memory to your mutable array by allocing in inside your Parent.m init method:

self.myArray = [NSMutableArray array];

answered Jan 27, 2014 at 0:18

Abhinav

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Comments

Duncan C · Accepted Answer · 2014-01-27 00:14:31Z

0

There is no need to use "super.myArray". All properties defined in a parent class are properties of the subclass. You could just as well use "self.myArray" They are the same object.

My guess is that you are actually dealing with 2 different objects. Post the code that creates your object, as well as the complete method that contains the line

[self.myArray addObject:myString];

Also add a log of self to both sections of code:

[self.myArray addObject:myString]; 
NSLog(@"In %s for %@, self.myArray = %@", __PRETTY_FUNCTION__, self, self.myArray);

And

- (void)myMethod 
{
  NSLog(@"In %s for %@ array = %@", __PRETTY_FUNCTION__, self, self.myArray); //always returns 0
}

answered Jan 27, 2014 at 0:14

Duncan C

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