Built-In Class object instance ruby data hash thread ((Ruby)) interpreter built-in class which the structure of ((instance|Instance)) is different from usual ((object|Object)). To define the class which inherits these classes is not recommended. A built-in class of ((Ruby)) is as follows: * Array * Bignum * Class * ((Data)) * FalseClass * File * Fixnum * Float * ((Hash)) * IO * ((MatchingData)) * Module * NilClass * Proc * Regexp * String * Struct * ((Thread)) * TrueClass