Which Ruby classes support .clone?

Question

Ruby defines #clone in Object. To my suprise, some classes raise Exceptions when calling it. I found NilClass, TrueCla

Answer 1

Fixnum is a special class given special treatment by the language. From the time your program launches, there is precisely one Fixnum for every number that the class can represent, and they're given a special representation that doesn't take any extra space — this way, basic math operations aren't allocating and deallocating memory like crazy. Because of this, there cannot be more than one 42.

For the others, they all have one thing in common: They're singletons. There's only one instance of a singleton class by definition, so trying to clone it is an error.