How to obtain the object corresponding to a given singleton (or eigen) class in Ruby?

Question

Suppose the following code:

class A
end

a = A.new

As = class << a
    self
end

# or:
# As = a.singleton_class

Is there some way to get a from As?

Answer 1

I think it is simple:

ObjectSpace.each_object(As).first
# => #<A:0x000000029a7c50>

Answer 2

Here's a trick for you:

class A
end

a = A.new

As = a.singleton_class

a2 = ObjectSpace.each_object.select{|obj| obj.singleton_class == As}.first

a2 == a # => true

Answer 3

This is just educated guessification on my part so YMMV, but I think "a" needs to be defined as a class variable (ie. "@@a = A.new") and then you would have the appropriate accessor method to return the class instance.

That being said, however, have you considered using the Singleton module (assuming you're on 1.9.3)?

Answer 4

The solution by sawa does not work for eigenclasses of classes. A universal method can be obtained via Object#to_s and Module#to_s methods, by parsing the returned string, see atalon.cz. However, this solution is not 100% reliable, because in Ruby, constants can be reassigned / deleted.