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Re: exception in class derived from NSMutableDictionary

Robert,

Your problem comes from the fact that you haven't correctly subclassed NSMutableDictionary.
You must take into account the fact that NSMutableDictionary is a class cluster (i.e., an abstract class providing no storage and only some methods, but not all). You have to provide storage (i.e., manage the internal state of the object) and to implement a set of so called "primitives methods" in your subclass. Here is the list of the primitive methods of NSMutableDictionary, extracted from the documentation: count, objectForKey:, keyEnumerator:, setObjectForKey:, removeObjectForKey:.

This may surprise you to learn that NSMutableDictionary is an abstract class because, as you pointed, you are able to instantiate NSMutableDictionary with [[NSMutableDictionary alloc] init]. But if you look closer, you will see that this does not return a direct instance of NSMutableDictionary, but an instance of a private concrete subclass of NSMutableDictionary (a subclass which provides storage and implements the primitives methods).

In order to master the subclassing of NSMutableDictionary (and many other classes in Cocoa), you should read this:
http://developer.apple.com/documentation/Cocoa/Conceptual/Foundation/ Concepts/ClassClusters.html

Contrary to popular belief, subclassing a class cluster like NSMutableDictionary is very easy and provides a correct way to implement your own storage if needed, which is what we want from a top notch object framework like Cocoa. In your case, look in details at the section "A Composite Object" in the class cluster documentation.

Best,

Phil

> I have a class that I have derived from NSMutableDictionary -
> solely for the purpose of being able to add items using a custom
> method. If I execute either of the first two pieces of code, they
> work fine. If I execute the third, I get an exception on the count
> method with the following message:
>
> 2003-07-30 11:47:27.964 V360Locator[1766] An uncaught exception was
> raised
> 2003-07-30 11:47:27.965 V360Locator[1766] *** -count only defined for
> abstract class. Define -[VideoSensorCollection count]!
> 2003-07-30 11:47:27.965 V360Locator[1766] *** Uncaught exception:
> <NSInvalidArgumentException> *** -count only defined for abstract
> class. Define -[VideoSensorCollection count]!
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