Question

Coding for iOS apps, I find there is potential reason to prohibit inheriting NSNumber, NSNull, …blabla.

If I try to do this, compiler does not fail but running will crash at once without any usage info, does anyone know the reason?

Answer 1

Subclassing Notes

As with any class cluster, if you create a subclass of NSNumber, you
have to override the primitive methods of its superclass, NSValue.
Furthermore, there is a restricted set of return values that your
implementation of the NSValue method objCType can return, in order to
take advantage of the abstract implementations of the non-primitive
methods. The valid return values are “c”, “C”, “s”, “S”, “i”, “I”,
“l”, “L”, “q”, “Q”, “f”, and “d”.

More precisely, I believe it’s because you never actually have an instance of NSNumber. Objective-C has the interesting property that the initializer of a class can actually change the object to which self points (this might actually be true of any method, but I’ve only ever heard of it being used in the init methods). I believe that NSNumber does this very thing.

See also the documentation on Class Clusters.