Understainding Singleton classes is extremely important for anyone programming in Ruby. Singleton classes explain how the concept of class/static method works in Ruby. It also explains how you can attach standalone methods to a particular object.

There are certain methods (like methods, singleton_methods, instance_methods, object_id) which prove extremely useful in getting the details of the class or understanding what actually happens in the background.

Firstly, how are class methods implemented in Ruby?

class/static methods are nothing but the methods that reside in the singleton class of any instance. For example consider the class below:

class User
  attr_accessor :name, :email

  def initialize(name, email)
    self.name = name
    self.email = email
  end

  def details
    "#{name} - #{email}"
  end

  def self.is_human?
    true
  end
end

User.singleton_methods ## returns [:is_human?]

Whenever you add a static method in the class, it gets attached to a singleton class in ruby

On a side note, there are three ways to add singleton methods. This is why it is generally told that there are multiple ways to do a certain thing in Ruby.

 # method 1
class User
  def self.add_singleton_method1
    "added"
  end
end


 # method 2
def User.add_singleton_method2
  "added"
end

 # method 3
class User
  class << self
    def add_singleton_method3
      "added"
    end
  end
end

User.singleton_methods # returns [:add_singleton_method1, :add_singleton_method2, :add_singleton_method3]

You can validate the presence of singleton class of User by executing:

User.object_id
User.singleton_class.object_id ## both the ids are different

Similarly, you can add singleton methods to the instance of the User class. And those methods can only be accessed by that particular instance of the class and no other. Singleton class should ideally be shown when you call ancestors on any instance, but it doesn’t appear as it is anonymous by default and can’t be accessed by any method so easily.