Better Know a Ruby Thing: Singleton Classes(noelrappin.com)

  private
  def self.foo
    # ...
  end

  def x.foo
    # ...
  end

  private_class_method def self.foo
    # ...
  end

  def self.foo
    # ...
  end
  private_class_method :foo # `def` will return a symbol of the defined method name so the above syntax is usually sufficient.

  def x.foo
    # ...
  end
  x.singleton_class.send :private, :foo # Class#private is a private method(!) so must be called using Object#send.

  class << x
  private
    def foo
      # ...
    end
  end

  class User
    class << self
      def create_from_data
      end
    end
  end

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