methods.rb

class Demo
  def self.a_class_method
    puts "Running a class method, self is: #{self.inspect}"
  end

  def an_instance_method
    puts "Running an instance method, self is: #{self.inspect}"
  end
end

Demo.a_class_method # class methods can be run on the class directly
demo_instance = Demo.new # .new creates an instance of Demo
demo_instance.an_instance_method # instance methods are run on instances of the class


# Expanding on the above code

class Demo
  class << self
    def another_class_method
      puts "Running a class method, self is: #{self.inspect}"
    end
  end
end

Demo.another_class_method


# The class method was defined as an instance method - so how is it that it can be called on the Demo class?
# The explanation is that class << self opens a class, like class Demo does. The class which is opened is
# Demo.singleton_class. Demo is an instance of Demo.singleton_class

class Demo
  class << self
    puts "self in the context of the singleton class: #{self}"
    puts "Demo.singleton_class == self # => #{Demo.singleton_class == self}"
  end
end

# Even thous Demo.class returns Class, the singleton_class technically sits in front of that in the inheritance chain:
puts "Demo.is_a?(Demo.singleton_class) # => #{Demo.is_a?(Demo.singleton_class)}"

# So class methods are really still instance methods - instance methods on the singleton_class of the class they're called on