This gets a bit tricky.

Suppose you add instance specific behavior to the pirate object. There now exists a Singelton class ‘pirate. A legitimate question is, “What is the class of ‘pirate?” If the world were sane the answer would be ”pirate, or Singleton class of the Singleton class of pirate. We know that ‘pirate’s super pointer points to Pirate, and, since ”pirate is a Metaclass (the class of the ‘pirate class), it’s super pointer should point to the Metaclass of ‘pirate’s super class. That would be ‘Pirate or Metaclass of Pirate. Another way to say this is, ” ‘pirate subclasses Pirate so ”pirate subclass ‘Pirate”. The __attached__ hack comes in because Ruby takes a shortcut. When it creates ‘pirate instead of setting it’s metaClass (or klass in MRI) to ”pirate, it sets it directly to ‘Pirate. This way method dispatch continues to work normally, and messages sent directly to ‘pirate (class methods) are matched against methods found in ‘Pirate. If you decide to define instance specific behavior on ‘pirate, the interpreter will grab the class of ‘pirate and find a Singleton class there, ‘Pirate, that is not attached to ‘pirate (it’s attached to the Pirate class), so the interpreter knows to create a new Metaclass, ”pirate, on the fly whose super pointer is ‘Pirate, and all is well in the world. The madness pretty much stops there. Ruby goes circular at that point, with the class of ”pirate being ”pirate. This happens in some code I elided from the makeMetaClass source in my post. It’s all pretty interesting, but so fringe that I don’t think it’s really worth people cooking their noodles over.

//RubyObject:217
if (this instanceof RubyClass && isSingleton()) { // could be pulled down to RubyClass in future
klass.setMetaClass(klass);
klass.setSuperClass(((RubyClass)this).getSuperClass().getRealClass().getMetaClass());
} else {
klass.setMetaClass(superClass.getRealClass().getMetaClass());
}

Hi Lopex,

Will definitely spend some time looking about in trunk. Thanks for reading the blog and all your JRuby work. Can you tell me under what circumstances __attached__ is null?

John Hume: the __attached__ thing might be also a null, in general it acts as a back pointer in singleton class chain.