Subclassing NSDictionary not working

Question

I have a dictionary that I read from a plist. I want to create a subclass of NSDictionary to implement something like the following, so that I can avoid using @"key name" everywhere in my source code:

@interface MyDict{
}
-(NSString*) textString;
@end

@implementation MyDict
-(NSString*) textString {
    return [self objectForKey:@"textString"];
}
@end

In my other method:

MyDict *d = ... // something i read from plist
NSString *str = [d textString];

When I call the method, the app crashes because of "unrecognized selector textString". What is wrong here?

Answer 1

Just assigning an NSDictionary to a MyDict pointer doesn't make it a MyDict instance.

One way you can do this would be to create a category to add your method to NSDictionary. See http://developer.apple.com/library/mac/#documentation/Cocoa/Conceptual/ObjectiveC/Chapters/ocCategories.html#//apple_ref/doc/uid/TP30001163-CH20-SW1 for info.

Answer 2

Your class has no superclass. Also, the conventional wisdom is that it's very difficult to subclass NSDictionary because it is an class cluster . You don't actually get an NSDictionary back when you:

NSDictionary * myDict = [NSDictionary dictionary];

You get a private subclass ( NSCFDictionary in this case).

You might want to try defining your own dictionary keys, the way Apple does:

NSString * const MyWonderfulUnicornKey = @"MyWonderfulUnicornKey";