gistfile1.rb

require 'fiddle'

class Object
  def cast(as)
    # Make sure iv table of the new class gets initialized.
    as.new.send(:instance_variable_set, :@nil, nil)
    rbasic = Fiddle::Pointer.new(__id__ <<  1)
    as_klass = Fiddle::Pointer.new(as.__id__ << 1)
    
    # Ruby guarantees sizeof(VALUE) == sizeof(void *)
    rbasic[Fiddle::SIZEOF_VOIDP, Fiddle::SIZEOF_VOIDP] = as_klass.ref.to_str(Fiddle::SIZEOF_VOIDP)
  end
end 

class Base
  def initialize
    @ivar = 42
  end
  
  attr_reader :ivar
end

class Derived < Base
  def initialize
    @ivar = 23
  end
  
  def bark
    puts @ivar
  end
end

obj = Base.new
obj.cast(Derived)
obj.bark