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Can You Really Be Your Own Grandpa? (Inheritance problem)

I subclassed NSDictionary to create OrderedDictionary (which is a dictionary that keeps the keys in the order they are recorded) and then I subclassed OrderedDictionary to produce MutableOrderedDictionary. So:

NSDictionary->OrderedDictionary->MutableOrderedDictionary

As would be expected, this code:

void doSomethingToADictionary(NSMutableDictionary* mutableDict)
{
...
}

MutableOrderedDictionary* dict = [MutableOrderedDictionary dictionary];

doSomethingToADictionary(dict);

causes this error:

type `NSMutableDictionary' is not a base type for type `MutableOrderedDictionary'

BUT this is the kicker (unless I am missing something): this following code does work when it still shouldn't, since NSDictionary doesn't inherit from NSMutableDictionary:

void doSomethingToADictionary(NSMutableDictionary* mutableDict)
{
...
}

NSDictionary* dict = [MutableOrderedDictionary dictionary]; //dict, of course,
really //represents a //MutableOrderedDictionary

doSomethingToADictionary(dict);

Why does this work?
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