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!

Answer 1

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];
}

Answer 2

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.