What is the internal design of Ruby's object model (hidden class + singleton class system)?

Ruby uses a hybrid object model with singleton classes, class hierarchy chains, and internal object shape optimizations.

Every Ruby object can have a hidden singleton class (eigenclass) that stores per-object methods. Classes themselves are objects, and inheritance is represented via ancestor chains. Method lookup traverses singleton class first, then class, then modules, then superclass. Internally, CRuby optimizes instance variable access using object shapes and offsets.