# Singleton Method Questions

**URL:** <https://www.ruby-forum.com/t/singleton-method-questions/100498>\
**Category:** Ruby\
**Created:** [February 3, 2006, 1:04am UTC](https://www.ruby-forum.com/t/singleton-method-questions/100498 "2006-02-03T01:04:27Z")\
**Posts on this page:** 2\
**Page:** 1

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**Author:** ![unknown](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/u/4bbf92/32.png) [@unknown](https://www.ruby-forum.com/u/unknown)\
**Post date:** [February 3, 2006, 1:04am UTC](https://www.ruby-forum.com/t/singleton-method-questions/100498/1 "2006-02-03T01:04:27Z")

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I’ve been thinking about singleton methods some more. The current  
implementation  
tucks away singleton methods in a class object. Method lookup is  
subsequently  
altered to consult the singleton class of the receiver (if it exists).

Is there a particular reason why a class object is used for this purpose  
rather than a module?

My motivation for this question was the observation that the  
semantics of a  
singleton class seem closer to a module than a class:

- method lookup is altered like a module mixin, but  
the inheritance structure is not modified nor is the change  
visible via ancestors (which makes sense because ancestors is  
only defined for a class not for an object but which suggests that  
ancestors could be defined in Kernel and return an array which  
includes the singleton classes for the object)

- if B is a subclass of A then method lookup for B.x consults  
the singleton class of B and the singleton class of A _as if_  
the singleton class of B included the singleton class of A  
even though the singleton class of B is _not_ a subclass of the  
singleton class of A. (i.e., method lookup is like included modules  
not like subclasses)

- the singleton class doesn’t have an instance (in the sense that  
there is  
no object x such that x.class references the singleton class).  
Calling  
new on a singleton class generates an exception. This is like a  
module.

- singleton classes don’t really participate in the inheritance  
hierarchy  
or perhaps it is more accurate to say that they participate in an  
extremely  
constrained way (superclass returns the singleton class of Class,  
yet I  
can’t come up with an example that would cause a method to be  
looked up  
in the singleton class of Class other than a direct message send  
to Class).

I think I’ve got the details correct but I welcome any corrections/  
clarifications.

Gary W.

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**Author:** ![unknown](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/u/4bbf92/32.png) [@unknown](https://www.ruby-forum.com/u/unknown)\
**Post date:** [February 3, 2006, 8:18am UTC](https://www.ruby-forum.com/t/singleton-method-questions/100498/2 "2006-02-03T08:18:54Z")

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Well, I just happened to be mucking about with that sort of thing and  
I think you might have an issue where if they were modules, you’d have  
to deal with being able to call foo.metaclass.extend\_object on another  
object, which then makes it pretty trivial to have it interacting in  
the inheritance hierarchy.

Incidentally, does anyone know of a way to do that sort of object  
prototyping thing under 1.8.4? It apparently used to be possible in  
1.8.2