iOS4 issue with dictionary

[matsoftware]

Hi everybody. I've update the SDK from 3.1.3 to 4.0. I opened a project (and I didn't make any change on it), then I ran it on simulator and it crashed with this error on console:

*** Terminating app due to uncaught exception 'NSInvalidArgumentException', reason: '-[NSCFString objectForKey:]: unrecognized selector sent to instance 0x5f8b2d0'
...
28  CoreFoundation                      0x0248eff4 __CFRUNLOOP_IS_CALLING_OUT_TO_A_SOURCE1_PERFORM_FUNCTION__ + 52
	29  CoreFoundation                      0x023ef807 __CFRunLoopDoSource1 + 215
	30  CoreFoundation                      0x023eca93 __CFRunLoopRun + 979
...

This error occurs when I use the dictionary: objectForKey method. XCode gives me no warning about any obsolete method or stuff like that.

Simply, It doesn't work with 4.x firmware. Any idea about it? Thank you :-)

[ChrisL]

The dictionary is probably being deallocated prematurely and its pointer left dangling. You can tell this because the message is actually being received by a string ([NSCFString objectForKey:]) instead of an NSDictionary. This usually happens because you've failed to retain the dictionary when you should have, or you've released the dictionary before you were actually done with it. In the meantime, the OS has reclaimed the deallocated memory and put something else in its place, and so your NSDictionary pointer now points to something else -- in this case, a string. To fix this, you need to go back through your code and make sure you're retaining and releasing the dictionary at the proper times.

The fact that this bug seemed to appear out of nowhere when changing something about the platform is pretty typical. Dangling pointers and other memory management bugs result in undefined behavior, which means that sometimes things can appear to be working normally even when there's a logic error in the code. In other words, the bug was probably always there, but it was just a coincidence that it didn't cause a crash under 3.x.

[matsoftware]

Thank you for your fast reply :-)

I've found in my code that this error occurs when I retrieve data from a .plist file. (I have a list of element that a user can manage by adding, moving, editing data - like a list of book - and this list is saved in the Documents folder of the App). The code I use to retrieve data is the following:

+ (NSString *)pathForDocumentWithName:(NSString *)documentName
{
   NSArray *paths = NSSearchPathForDirectoriesInDomains(NSDocumentDirectory,
                                                         NSUserDomainMask, 
                                                         YES);
    NSString *path = [paths objectAtIndex:0];
    
    return [path stringByAppendingPathComponent:documentName];
}

- (NSMutableArray *)displayedObjects
{
    if (_displayedObjects == nil)
    {
        NSString *path = [[self class] pathForDocumentWithName:@"Books.plist"];
        NSArray *booksDicts = [NSMutableArray arrayWithContentsOfFile:[path stringByStandardizingPath]];
        
        if (booksDicts == nil)
        {
            NSLog(@"Unable to read plist file at path: %@", path);
            path = [[NSBundle mainBundle] pathForResource:@"Books"
                                                   ofType:@"plist"];
            booksDicts = [NSMutableArray arrayWithContentsOfFile:path];
        }
        
        _displayedObjects = [[NSMutableArray alloc]
                             initWithCapacity:[booksDicts count]];
          
        for (NSDictionary *currDict in booksDicts)
        {
            Book *books = [[Book alloc] initWithDictionary:currDict];
            [_displayedObjects addObject:books];
        }
    }
    
    return _displayedObjects;
}

where "Book" is the class that manage the list. If there is no file (like the first use) I don't have problem with the calling of the class "Book" and the dictionary: objectForKey . The error occurs in the bold row, and I don't know how figure it out.