From: nobu.nokada@...
Date: 2002-01-27T13:56:53+09:00
Subject: Re: Subrange of String subclass => invalid object

Hi,

At Sat, 26 Jan 2002 14:20:32 +0900,
Bob Alexander <bobalex@attbi.com> wrote:
>   -  It's a pretty good bet that leaving the instance variable uninitialized
> does not result in a valid object.

Agree.

>   -  We have no way of knowing whether copying the value (77) from the
> original instance results in a valid object. Maybe it's a hash of the
> string, or an object sequence number, in which case it should be something
> different from 77.
> 
> I don't know of a foolproof way for a superclass to create a valid instance
> of the subclass without knowledge of the subclass.

How about this?

  class S < String
    def [](*)
      dup.replace(super)
    end
  end

It works as this right now.

  class S < String
    def [](*)
      type.allocate.replace(super)
    end
  end

> A simple answer that probably most folks won't like is: the superclass could
> return an instance of itself (seems natural enough!). The subclass can then
> do what it wants with it (in an overriding method), including returning a
> new subclass instance, modifying the existing self instance, returning the
> superclass instance, or anything else it chooses. This is a bit more work
> for the subclasser, but it is airtight.

Isn't it latter of [ruby-talk:32462]?

-- 
Nobu Nakada