metaclass.rb

foo = Class.new  # You now have an anonymous class, which is just an instance of the Class class.

# instance_exec sets `self` to the given instance (<foo>, in this case, our anonymous class)
# for the duration of the block
foo.instance_exec do
  define_method :bar do
    "Hello from an instance of foo! I am #{self.inspect}"
  end

  # Here, we're going to step into <foo>'s metaclass, which is the class which defines singleton instance methods for <foo>
  class << self
    def baz
      "Hi from foo's metaclass, I am #{self.inspect}"
    end
  end
end

puts foo.baz
puts foo.new.bar


# => Hi from foo's metaclass, I am #<Class:0x649d209a>
# => Hello from an instance of foo! I am #<#<Class:0x649d209a>:0x6adca536>

You can see that <foo> is an instance of the anonymous metaclass <Class:0x649d209a>!