From: David Alan Black <dblack@...>
Date: 2002-07-05T08:45:06+09:00
Subject: Re: self_parent

Hi --

On Fri, 5 Jul 2002, Tom Sawyer wrote:

> class Customer
>   attr_reader :name, :address
>   def initialize(name, address)
>     @name = name
>     @address = Address.new(address)
>   end
>   def pretty_name
>     return @name.upcase
>   end
> end
>
> class Address
>   def initialize(address)
>     @address = address
>   end
>   def print_label
>     puts self_parent.pretty_name
>     puts @address
>   end
> end
>
> c = Customer.new("John Doe", "111 Test Ln.\n Santa Fe, NM 87501")
> c.address.print_label

This introduces a dependency between objects, though, based on a kind
of "self" stack, which won't necessarily mesh with the actual design
and behavior of the classes and objects.

For example, let's say you wanted to refactor/rewrite Customer like
this:

  class Factory
    def Factory.make(thing,*args)
      const_get(thing).new(*args)
    end
  end

  class Customer
    def initialize(name, address)
      @name = name
      @address = Factory.make("Address", address)
    end
    ...
  end

Not a very elegant example -- but the point is, now you're two "self"s
away, instead of one, but your Address objects are hardcoded to look
one "self" away.

I know Ruby allows for a lot of introspection... but I think relying
on this would constrain code and code growth significantly.

> no need to pass self and explicitly store it. so there's a fictional
> example for you. in the real world there are many cases of such a
> parent-child relationship. in fact i think REXML has an explicit Parent
> class it uses to glue such realtionships together. hiearchy structures
> too (like a tree) are another area in which a parent-child relationships
> of this sort are used. and i'm sure there are many others.

REXML is specifically modeling the parent/child relationships of XML
in that class, though (I think).  That's different from a
language-level (I mean Ruby, not XML) description of one object as the
"parent" of another because the second object is created by a method
of the first object.


David

-- 
David Alan Black
home: dblack@candle.superlink.net
work: blackdav@shu.edu
Web:  http://pirate.shu.edu/~blackdav