Determine Class from Eigenclass

Question

In Ruby, getting the eigenclass of a class Foo is a simple as

eigenclass = class << Foo; self; end
#=> #<Class:Foo>
eigenclass = Foo.singleton_class #2.1.0
#=> #<Class:Foo>

I'm interested in the inverse operation: getting the owner of the eigenclass from the eigenclass itself:

klass = eigenclass.owner
#=> Foo

I'm not sure if this is possible, given that the eigenclass is an anonymous subclass of Class, so Foo appears nowhere in its inheritance hierarchy. Inspecting the method list of the eigenclass isn't encouraging either. eigenclass.name returns nil. The only thing that gives me hope this is possible:

Class.new # normal anon class
#=> #<Class:0x007fbdc499a050>
Foo.singleton_class
#=> #<Class:Foo>

Clearly, the eigenclass's to_s method knows something about the owner, even if this information is hardcoded when the eigenclass is instantiated. Therefore the only method I'm aware of is some hacky Object.const_getting from that like

Object.const_get eigenclass.to_s[/^#\<Class\:(?<owner>.+)\>$/, :owner]
#=> Foo

Answer 1

Use ObjectSpace.each_object passing it the singleton class to find all classes that match the given singleton class:

Klass = Class.new
ObjectSpace.each_object(Klass.singleton_class).to_a  #=> [Klass]

However, since a class’s singleton class inherits from its superclass’s singleton class, you’ll get multiple results if the class you’re trying to find has subclasses:

A = Class.new
B = Class.new(A)
B.singleton_class.ancestors.include?(A.singleton_class)  #=> true

candidates = ObjectSpace.each_object(A.singleton_class)
candidates.to_a  #=> [A, B]

Fortunately, classes/modules are sortable by their place in the inheritance tree (same order ancestors gives). Since we know all the results must be part of the same inheritance tree, we can take the max to get the correct class:

candidates.sort.last  #=> A
ObjectSpace.each_object(B.singleton_class).max  #=> B

Answer 2

Refining @BroiSatse's answer in a ruby-implementation-agnostic way,

class A; end
class B < A; end
class C < A; end
eigenclass = A.singleton_class
ObjectSpace.each_object(eigenclass).find do |klass|
  klass.singleton_class == eigenclass
end
#=> A

This is also reliable when handling branches in subclass trees, the only reason why @Andrew Marshall's elegant answer doesn't work.