Having trouble adding objects to NSMutableArray in Objective C

Question

I am using the iPhone SDK and have an issue doing something simple. I am trying to add an NSNumber object to an NSMutableArray instance variable. I tried adding NSNumber card to NSMutableArray viewedCardsArray, however without breaking, it does not get added to the array. Here is the code.


/////////////////////////////////////////////////////
// Inside the header file Class.h
@interface MyViewController : UIViewController {
   NSMutableArray *viewedCardsArray;
   //snip ...
}
@property (nonatomic, retain) NSMutableArray *viewedCardsArray;
@end

/////////////////////////////////////////////////////
// Inside the methods file Class.m
#import "StudyViewController.h"

@implementation StudyViewController
@synthesize viewedCardsArray
  //snip ...

- (IBAction)doShowCard {
   //snip ...
   NSNumber *cardIdObject = [[NSNumber alloc] initWithInt:(int)[self.currentCard cardId]];
   [viewedCardsArray addObject: cardIdObject];
   [cardIdObject release];
}

So this code executes, and does not seem to leak (according to the Leaks performance tool). However when stepping through the code, at no point does CardIdObject appear in viewedCardsArray.

Looking through SO, I know these basic questions are pretty common to ObjC newbies (like me) so apologies in advance!

Alex Rozanski · Accepted Answer · 2009-05-12 13:11:03Z

17

Have you initialized your viewedCardsArray? If not you need to somewhere - this is usually done in the init method for your class:

- (id)init
{
    self = [super init];
    if(self) {
        viewedCardsArray = [[NSMutableArray alloc] init];
    }
    return self;
}

Then it is released in the dealloc method:

- (void)dealloc
{
    [viewedCardsArray release];
    [super dealloc];
}

edited May 12, 2009 at 13:11

answered May 12, 2009 at 9:32

Alex Rozanski

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5 Comments

Peter Hosey Over a year ago

Don't forget to use the result of [super init]. It can return nil or a different object. Never assume that it returned your former self, because when it doesn't, it released it, and any further initialization you do of that now-dead object will probably cause a crash. The simplest way is if ((self = [super init])) { /*initialize here*/ } return self; (subject to your own style taste—for example, you can have the assignment before the condition if you want).

Alex Rozanski Over a year ago

Sorry I meant that line to be self = [super init]; but yes, that is a better approach

pchap10k Over a year ago

Thank you for your help everyone. I now appreciate the difference between a class declaration and instantiating a variable!

SundayMonday Over a year ago

I had myArray = [[NSArray alloc] init]; in my init method. However the child class wasn't calling init. It was calling initWithStyle. So my array initialization was not happening 8-)

dmgig Over a year ago

This was my mistake as well. I assumed that having it in the header and synthesizing it initialized the object. Obviously not!

Alastair Stuart · Accepted Answer · 2009-05-12 14:08:29Z

3

Perspx has outlined one way of initializing the array. However, you can also use the class methods provided by NSArray:

self. viewedCardsArray = [NSMutableArray array];

This can go in init or elsewhere.

Note: The object will be autoreleased.

answered May 12, 2009 at 14:08

Alastair Stuart

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2 Comments

Peter Hosey Over a year ago

The problem with that is that when you set your own properties in -init, you're sending messages (specifically, accessor messages) to a not-fully-initialized object (self). The same is true in reverse in -dealloc. This can become a problem if you implement a custom accessor that assumes that some other ivar has already been initialized and that the object in it (if any) is still valid.

Jim Dovey Over a year ago

If you're using synthesized properties, this is a perfectly valid way of doing things. There's Apple code which does this, for example, as well as the inverse: using self.someProperty = nil in the -dealloc method.

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