Why am I having trouble with a deep copy in Objective C?

Answer 1

Auto release on my dictionaries array is happening, thus my app crashing.

If the caller intends to keep the array around somewhere, it needs to retain it. Otherwise, it will crash when it tries to access the (now-deceased) object.

If the caller is going to store it in a property, it must use the self.dictionaries = […] syntax, not dictionaries = […]. The former is a property access, which calls the setter method; the latter is a direct instance variable assignment.

Coming back to your actual question, that of a deep copy: You need to get the sub-elements of every element and put them in each element's dictionary.

Basically, you need a recursive method (or a queue, but that's harder—file under premature optimization until you've proven you need it) that takes an element and returns a dictionary, and then you need to call this method on each of your element's child elements, and collect the results into an array and put that into the dictionary you're creating.

I would recommend making this recursive method an instance method of the element. Something like:

- (NSDictionary *) dictionaryRepresentation {
 NSMutableDictionary *attributeDictionary = [NSMutableDictionary dictionary];
 for (CXMLNode *attribute in attributes) {
  [attributeDictionary setObject:[attribute stringValue] forKey:[attribute name]];
 }

 NSArray *childElements = [self childElements];

 return [NSDictionary dictionaryWithObjectsAndKeys:
  attributeDictionary, @"attributes",
  [childElements valueForKey:@"dictionaryRepresentation"], @"childElements",
  nil];
}

Then you replace the loop in createArrayWithDictionaries:withXPath: with a similar valueForKey: message. I'll leave you to fill it in.

valueForKey: is Key-Value Coding's principal method. In both places, we're making use of NSArray's handy implementation of it.

(If the use of valueForKey: still doesn't make sense to you, you should read the KVC Programming Guide. KVC is vitally important in modern Cocoa, so you do need to read this sooner or later.)

Answer 2

OK, you don't need to retain the return value from createArrayWithDictionaries: since you're not keeping it around. The return value is autoreleased. I'd strongly recommend reading up on how autoreleasing works. You only retain things that you intend to keep around in your object.

Also, user is an NSArray. If you call [user valueForKey:@"name"], you'll get another NSArray of values representing the values of the name key for each of the objects in users. Furthermore, how is the name property on your object defined? If you declared it as copy or retain (I believe retain is the default if you don't specify it yourself), you don't need to copy or retain the value. Indeed, the accessor should always be responsible for doing the memory management, not the caller. If you wrote your own accessor (i.e. you didn't use the @synthesize keyword), you need to make sure you do the memory management there.

I'm guessing what you meant to write was something more like this:

    NSArray *user = [xmlParser createArrayWithDictionaries:dataAsXML withXPath:kUserXPath];
    if ([user count] > 0)
        self.name = [[user objectAtIndex:0] objectForKey:@"name"];

    [xmlParser release];

I think your troubles are stemming from a misunderstanding of how memory management works in Objective-C.

Hope this helps.