Acquire instance from metaclass

Question

Given a metaclass in ruby (or eigenclass or singleton_class), how would I find the instance of that class? To the best of my knowledge, metaclasses are intrinsically linked with a SINGLE instance of said metaclass. Therefore, there should be a way to find the instance of it, right?

For example...

class Example
  def self.do_something
    "Hello everyone!"
  end
end

p Example #should output 'Example'
p Example.singleton_class #should output '#<Class:Example>'

I want a method that would do the following:

def meta_do_something(meta)
  #invokes the do_something method on the object that meta references
end

meta_do_something(Example.singleton_class) #should output "Hello everyone!"

I've been searching all around, but I can find nothing... any ideas?

Answer 1

A class (Example in your case) is not an instance of its singleton class. It is a sub-class. Sub-classes are not instances of their super-classes, they're just...well, sub-classes. Instances are objects created with from SomeClass with SomeClass.new. There is a reason why Ruby makes meta-classes invisible. I guess your fundamental question is basically whether a super-class can know about its sub-class, and the answer is no, I'd imagine that would break a lot of OOP concepts (a dog is an animal, but not every animal is a dog).

It's true that do_something is an instance method of the Singleton class (all methods are instance methods in Ruby technically speaking) but the fact is that you cannot initialize a new Singleton class in a same way like you would initialize any other class. There's a reason why Ruby makes this class invisible (cannot be seen by calling .ancestors).

Edit: This is an (ugly) hack that does the job if you're looking to do this at all costs:

def meta_do_something(meta)
  actual_class = Object.const_get(meta.to_s[/(?<=#<Class:)[^>]+/])
  method_to_call =  meta.instance_methods(false)[0]
  actual_class.send(method_to_call)
end

meta_do_something(Example.singleton_class) #=> "Hello everyone!"

If you wanted to call all methods defined in the singleton class, you could do something like this (replacing the 3rd and 4th line):

meta.instance_methods(false).each do |method|
    actual_class.send(method)
end