dyn.rb

irb(main):010:0> self.class.const_set(:SomeClass,
irb(main):011:1*   Class.new(Ancestor) do
irb(main):012:2*     def initialize(var)
irb(main):013:3>       print "#{self.class} initialized with #{var}"
irb(main):014:3>     end
irb(main):015:2>   end
irb(main):016:1> )
=> SomeClass
irb(main):017:0> SomeClass.new("foo")
SomeClass initialized with foo=> #<SomeClass:0x67b77c>