Question

I’m getting this error when trying to see the contents of a NSMutableArray:

Program received signal EXC_BAD_ACCESS, Could not access memory.
Reason: KERN_PROTECTION_FAILURE at address: 0x00000021
0x94d5a688 in objc_msgSend ()

ViewController.h:

@interface PeopleViewController : UITableViewController {
    NSMutableArray *people;
}

@property (nonatomic, retain) NSMutableArray *people;

ViewController.m:

@implementation PeopleViewController

@synthesize people;

In viewDidLoad:

- (void)viewDidLoad {
    [super viewDidLoad];

    // initialize our people array with an autoreleased object
    people = [NSMutableArray array];

... Populate the people array with Person objects.

}

When I’m at the point where I’m modifying the contents of a cell in the tableview, I’m unable to access the people array in gdb when typing ‘po self.people’:

Person *person = [[Person alloc] init];
person = [self.people objectAtIndex: indexPath.row]; // <--- 'po self.people' called  
cell.textLabel.text = person.personName;

Any ideas why I can’t access it?

Answer 1

The line

people = [NSMutableArray array];

returns an autoreleased array that will be released on the next iteration of the current run loop. You should retain that:

people = [[NSMutableArray array] retain];

and of course release it in your dealloc method.

However: Apple engineers have often mentioned in conferences to avoid autoreleased instances like this whenever possible in the iPhone, for performance reasons. Try using alloc/init instead:

people = [[NSMutableArray alloc] initWithCapacity:1];

with the corresponding release in the dealloc method. In this case you don’t even need to retain (init returns an instance with a retain count of 1, which is what you need).

And justin’s comment is correct: you should do this instead:

Person *person = [people objectAtIndex:indexPath.row];
cell.textLabel.text = person.personName;

and this should work.