Question

I want to make a “Favorites” in my RSS reader. My RSS parser is parsing a RSS feed to NSMutableArray, and then object “item” is created from part of my rss (selected post).
My code:

//Creating mutable array and adding items:

    - (void)viewDidLoad {
        if (favoritedAlready == nil) {
            favoritedAlready = [[NSMutableArray alloc] init];
            [[NSUserDefaults standardUserDefaults] setObject:favoritedAlready forKey:@"favorites"];
            [[NSUserDefaults standardUserDefaults] synchronize];
            NSLog(@"избранное с нуля");
        }
    }
    - (void) addToFavorites {
        NSMutableArray* favoritedAlready = [[NSUserDefaults standardUserDefaults] objectForKey:@"favorites"];
        [favoritedAlready addObject: item];
        [[NSUserDefaults standardUserDefaults] setObject:favoritedAlready forKey:@"favorites"];
        [[NSUserDefaults standardUserDefaults] synchronize];

        NSLog(@"Добавлено в избранное. В избранном %i статей", [favoritedAlready count]);
    }

    //Removing items (another View)
    - (void)viewWillAppear:(BOOL)animated {
        [super viewWillAppear:animated];
        rssItems = [[NSUserDefaults standardUserDefaults] objectForKey:@"favorites"];
        [self.tableView reloadData];
        NSLog(@"Загрузилось избранное, %i избранных статей", [rssItems count]);
    }
    - (void)tableView:(UITableView *)tableView commitEditingStyle:(UITableViewCellEditingStyle)editingStyle forRowAtIndexPath:(NSIndexPath *)indexPath {
        NSLog(@"Номерок строчки в которой удаляемый объект %i", indexPath.row+1);
        [rssItems removeObjectAtIndex:indexPath.row];
        [self.tableView deleteRowsAtIndexPaths:[NSArray arrayWithObject:indexPath] withRowAnimation:UITableViewRowAnimationFade];

        [[NSUserDefaults standardUserDefaults] setObject:rssItems forKey:@"favorites"];
        [[NSUserDefaults standardUserDefaults] synchronize];
    }

It worked perfectly initially, but when I added and removed items, it started crashing.
Crash logs:
I added object to Favorites and removed it:

2011-09-25 20:14:44.534 ARSSReader[36211:11303] *** Terminating app due to uncaught exception 'NSInternalInconsistencyException', reason: '-[__NSCFArray removeObjectAtIndex:]: mutating method sent to immutable object'
*** Call stack at first throw:
(
    0   CoreFoundation                      0x015505a9 __exceptionPreprocess + 185
    1   libobjc.A.dylib                     0x016a4313 objc_exception_throw + 44
    2   CoreFoundation                      0x01508ef8 +[NSException raise:format:arguments:] + 136
    3   CoreFoundation                      0x01508e6a +[NSException raise:format:] + 58
    4   CoreFoundation                      0x01547dd1 -[__NSCFArray removeObjectAtIndex:] + 193
    5   ARSSReader                          0x000a0ced -[FavoritesView tableView:commitEditingStyle:forRowAtIndexPath:] + 173
    6   UIKit                               0x00876037 -[UITableView(UITableViewInternal) animateDeletionOfRowWithCell:] + 101
    7   UIKit                               0x0080b4fd -[UIApplication sendAction:to:from:forEvent:] + 119
    8   UIKit                               0x0089b799 -[UIControl sendAction:to:forEvent:] + 67
    9   UIKit                               0x0089dc2b -[UIControl(Internal) _sendActionsForEvents:withEvent:] + 527
    10  UIKit                               0x0089c7d8 -[UIControl touchesEnded:withEvent:] + 458
    11  UIKit                               0x0082fded -[UIWindow _sendTouchesForEvent:] + 567
    12  UIKit                               0x00810c37 -[UIApplication sendEvent:] + 447
    13  UIKit                               0x00815f2e _UIApplicationHandleEvent + 7576
    14  GraphicsServices                    0x01c91992 PurpleEventCallback + 1550
    15  CoreFoundation                      0x01531944 __CFRUNLOOP_IS_CALLING_OUT_TO_A_SOURCE1_PERFORM_FUNCTION__ + 52
    16  CoreFoundation                      0x01491cf7 __CFRunLoopDoSource1 + 215
    17  CoreFoundation                      0x0148ef83 __CFRunLoopRun + 979
    18  CoreFoundation                      0x0148e840 CFRunLoopRunSpecific + 208
    19  CoreFoundation                      0x0148e761 CFRunLoopRunInMode + 97
    20  GraphicsServices                    0x01c901c4 GSEventRunModal + 217
    21  GraphicsServices                    0x01c90289 GSEventRun + 115
    22  UIKit                               0x00819c93 UIApplicationMain + 1160
    23  ARSSReader                          0x00001e79 main + 121
    24  ARSSReader                          0x00001df5 start + 53
)
terminate called throwing an exception(gdb) 

But, if I added item, restarted app and then removed it, it worked perfectly.

I added item, restarted app, removed item, and tried to add new item:

2011-09-25 20:19:19.212 ARSSReader[36461:11303] *** Terminating app due to uncaught exception 'NSInternalInconsistencyException', reason: '-[__NSCFArray insertObject:atIndex:]: mutating method sent to immutable object'
*** Call stack at first throw:
(
    0   CoreFoundation                      0x015505a9 __exceptionPreprocess + 185
    1   libobjc.A.dylib                     0x016a4313 objc_exception_throw + 44
    2   CoreFoundation                      0x01508ef8 +[NSException raise:format:arguments:] + 136
    3   CoreFoundation                      0x01508e6a +[NSException raise:format:] + 58
    4   CoreFoundation                      0x01547cf1 -[__NSCFArray insertObject:atIndex:] + 209
    5   CoreFoundation                      0x01544c14 -[__NSCFArray addObject:] + 68
    6   ARSSReader                          0x00004b35 -[DetailsViewController addToFavorites] + 149
    7   UIKit                               0x0080b4fd -[UIApplication sendAction:to:from:forEvent:] + 119
    8   UIKit                               0x00a1dcc3 -[UIBarButtonItem(UIInternal) _sendAction:withEvent:] + 156
    9   UIKit                               0x0080b4fd -[UIApplication sendAction:to:from:forEvent:] + 119
    10  UIKit                               0x0089b799 -[UIControl sendAction:to:forEvent:] + 67
    11  UIKit                               0x0089dc2b -[UIControl(Internal) _sendActionsForEvents:withEvent:] + 527
    12  UIKit                               0x0089c7d8 -[UIControl touchesEnded:withEvent:] + 458
    13  UIKit                               0x0082fded -[UIWindow _sendTouchesForEvent:] + 567
    14  UIKit                               0x00810c37 -[UIApplication sendEvent:] + 447
    15  UIKit                               0x00815f2e _UIApplicationHandleEvent + 7576
    16  GraphicsServices                    0x01c91992 PurpleEventCallback + 1550
    17  CoreFoundation                      0x01531944 __CFRUNLOOP_IS_CALLING_OUT_TO_A_SOURCE1_PERFORM_FUNCTION__ + 52
    18  CoreFoundation                      0x01491cf7 __CFRunLoopDoSource1 + 215
    19  CoreFoundation                      0x0148ef83 __CFRunLoopRun + 979
    20  CoreFoundation                      0x0148e840 CFRunLoopRunSpecific + 208
    21  CoreFoundation                      0x0148e761 CFRunLoopRunInMode + 97
    22  GraphicsServices                    0x01c901c4 GSEventRunModal + 217
    23  GraphicsServices                    0x01c90289 GSEventRun + 115
    24  UIKit                               0x00819c93 UIApplicationMain + 1160
    25  ARSSReader                          0x00001e79 main + 121
    26  ARSSReader                          0x00001df5 start + 53
)
terminate called throwing an exception(gdb) 

Answer 1

In addToFavorites

NSMutableArray* favoritedAlready = [[NSUserDefaults standardUserDefaults] objectForKey:@"favorites"];

will return an NSArray (it makes no difference it you save a mutable version), not an NSMutableArray

You need to create a mutable version:

NSMutableArray* favoritedAlready = [[[NSUserDefaults standardUserDefaults] objectForKey:@"favorites"] mutableCopy];

You can obviously not add items to a non-mutable array. Also just casting the return value to a NSMutableArray is wrong, it may work or it may not but that is irrelevant.