Apart from this being the prime example for the argument for immutable data (i.e. just keep the whole state of your editor at every undo point somewhere; restor

Apart from this being the prime example for the argument for immutable data (i.e. just keep the whole state of your editor at every undo point somewhere; restore to previous states when undoing) the "OO way" doesn't even need OO features, i.e. see typeclasses or traits.