iPhone NSMutableArray and NSKeyedUnarchiver unarchiveObjectWithFile release oddity

Question

I archive an array (NSMutableArray) of custom objects that implement the . Once i load it froma file to a retaining property @property (nonatomic, retain) NSMutableArray *buddies; the release count of the object is 2 (correct, it's 1of autorelease + 1 of retain of the property) but then noone releases it and the retain count becames 1, so when i release it i get -[__NSArrayM retainCount]: message sent to deallocated instance (i think because the 1 retain count is the autorelease)

Here's the full code:

BuddieListViewController.h

#import <UIKit/UIKit.h>
#import "Buddie.h"

@interface BuddieListViewController : UITableViewController {
    IBOutlet NSMutableArray *buddies;
    [...]
}
[...]
@property (nonatomic, retain) NSMutableArray *buddies;
[...]
@end

BuddieListViewController.m

#import "BuddieListViewController.h"
#import "Buddie.h"
#import "PreviewViewController.h"

@implementation BuddieListViewController

@synthesize buddies;
[...]
- (id)initWithStyle:(UITableViewStyle)style
{
    self = [super initWithStyle:style];
    if (self) {
        [self loadFromDisk];
    }
    return self;
}

- (void)loadFromDisk {
    NSArray *paths = NSSearchPathForDirectoriesInDomains(NSDocumentDirectory, NSUserDomainMask, YES);
    NSString *documentsPath = [paths objectAtIndex:0];
    NSString *appFile = [documentsPath stringByAppendingPathComponent:@"BuddieArchive.ark"];
    NSFileManager *fileManager = [NSFileManager defaultManager];
    if ([fileManager fileExistsAtPath:appFile]) {
        self.buddies = [NSKeyedUnarchiver unarchiveObjectWithFile:appFile];
        NSLog(@"1- buddies retain count %d (should be 2, 1 + 1autorelease)", [buddies retainCount]);
    } else {
        self.buddies = [NSMutableArray arrayWithCapacity:1];
    }
}
[...]
- (IBAction)cancelledBuddie:(NSNotification *)notification
{
    [editingBuddie release];
    NSLog(@"2- buddies retain count %d (should be 2, 1 + 1autorelease)", [buddies retainCount]);
    [buddies release]; 
    [self loadFromDisk];
    [self.tableView reloadData];
}

Has anyone some idea of why this happens?

Answer 1

I can't say it better than this:

The number returned by retainCount is useless.

Don't rely on it for anything. Use the Leaks tool in Instruments to determine if you're leaking objects.

---

If you're crashing, it's most likely that you have a zombie. See this video to find out how to use Instruments to find zombies.

Answer 2

Rather than releasing buddies why not just do a [self.buddies removeAllObjects] at the beginning of loadFromDisk.

Answer 3

If you need to nullify the array, use the property accessor to set it to nil:

self.buddies = nil;

The synthesized implementation takes care of the memory management issues. Try to avoid sending -retain/-release messages directly to instance variables wherever possible and instead allow the property accessors to take care of things for you. It'll save you a lot of trouble.