objective c - Using class as key in NSDictionary

Question

I'm writing a contextual "factory" that will maintain a dictionary of converter/acting objects which inherit from some Converter class. This class has a method:

- (Class)classResponsibility

Or something similar, such that a StringConverter class would implement the method as:

- (Class)classResponsibility {
    return [NSString class];
}

Then to store that converter in the dictionary, I had hoped on doing something like:

[converters setValue:stringConverter forKey:[stringConverter classResponsibility]];

But the compiler complains that the type "Class" is an invalid parameter type for argument 2 of the setValue:forKey: method. I had wanted to avoid setting the key as the Class's name ("NSString"), but if that's the best solution than I'll go with it.

question from:https://stackoverflow.com/questions/730076/using-class-as-key-in-nsdictionary

Answer 1

You're using setValue:forKey: which only takes NSStrings as keys. you should be using setObject:forKey: instead. A class object (pointers to class objects can be passed as type Class) is a full-fledged Objective-C object (a class object is an instance of its meta-class, and you can use all the NSObject methods on a class object; read more about meta-classes here), so they can be used anywhere objects are used.

Another requirement for keys of a dictionary is that they support copying (i.e. have the copyWithZone: method. Do class objects support this method? In fact, it does. The NSObject class defines a class method +copyWithZone:, whose documentation explicitly says that it "lets you use a class object as a key to an NSDictionary object". I think that's the answer to your question.