Retrieve a Ruby object from its singleton class?

It is possible to access a singleton class from a Ruby object with:

some_object.singleton_class

Is it possible to do the reverse operation : access the original object when inside the singleton class?

class << some_object
  # how to reference some_object without actually typing some_object?
end

I wanted to DRY this method:

class Example
  PARENTS = []
  class << PARENTS
    FATHER = :father
    MOTHER = :mother
    PARENTS.push(FATHER, MOTHER)
  end
end

and tried to replace PARENTS inside the class with something more generic.

Solution

Class#attached_object (Ruby 3.2+)

Starting from Ruby 3.2, there is a Class#attached_object method:

Returns the object for which the receiver is the singleton class.

Raises an TypeError if the class is not a singleton class.

For example:

class Foo; end

Foo.singleton_class.attached_object     #=> Foo
Foo.attached_object                     #=> TypeError: `Foo' is not a singleton class
Foo.new.singleton_class.attached_object #=> #<Foo:0x000000010491a370>
TrueClass.attached_object               #=> TypeError: `TrueClass' is not a singleton class
NilClass.attached_object                #=> TypeError: `NilClass' is not a singleton class

Sources: