Question

I would like to know is there any ways to check is singleton class of an
object already created?

ex: obj.singleton_class_defined?

Answer 1

The singleton class of an object is always defined. In 1.8.7 you can use singleton_methods to see if an object already has associated singleton methods:

>> foo = ''
=> ""
>> foo.singleton_methods
=> []

1.9.2 (possibly also earlier 1.9s, I can’t remember) also has a method called singleton_class, which saves you from doing the class << self; self ; end thing we all got used to:

>> foo.singleton_class #=> #<Class:#<String:0x00000100ba5648>>

Edit:

Since you tagged this with “object-model”, I also wanted to recommend the following link:

http://www.hokstad.com/ruby-object-model.html

To quote from there:

A meta-class is for all practical
purposes an actual class. It is an
object of type Class. The only thing
“special” about a meta-class is that
it is created as needed and inserted
in the inheritance chain before the
objects “real” class. So inside the
MRI interpreter object->klass can
refer to a meta-class, that has a
pointer named “super” that refers to
the next class in the chain. When you
call object.class in MRI, the
interpreter actually “skips” over the
meta-class (and modules) if it’s
there.