From: dblack@...
Date: 2002-09-22T12:43:10+09:00
Subject: self as method argument revisited

Hello --

[Disclaimer: this isn't intended as a general solution, just something
I've been working on.]

A while ago transami brought up the question of an object needing
access to the object that created it, and there was discussion of
techniques including sending self as an argument.  I've been working
on a case of the underlying problem he was talking about, and I
thought I'd share my (current) solution.  Comments welcome.

The big object is a (simple) database image.  It creates Row objects
from arguments to an #insert method, and populates itself hash-wise
with the values.

There are also field names.  A Row object needs to know its own field
names, so that one can do things like:

  r = db["Black"]   # r is a Row object
  r["first_name"]   # => David

Originally I had been creating an array of field names for every row,
but that seemed very clunky.  Then I got an idea....

When the database object is created, it creates a new class, which
inherits from an existing Row class (DBDBD::Row).  That new class has
an instance variable (i.e., its virtual class defines an accessor
method) called @row_class.  The new database object assigns itself to
@db in that Row class's virtual class:

  class DBDB
    def initialize(...)
      ...
      @row_class =
	class << self
	  class Row < DBDBD::Row
	    class << self
	      attr_accessor :db
	    end
	    self
	  end
	end
      @row_class.db = self
    end
    ...
  end

The Row objects already know that they can get their fields by doing:

  type.db.fields

So everybody's happy :-)  The database object does not have to pass
itself as an argument when initializing rows, and the rows don't have
to carry around a copy of the field names (which perhaps they were
doing only by reference, so it wasn't a storage issue, but it was
still awkward).

I just thought I'd share this since it reminded me so exactly of the
problems transami and others had been talking about.


David

-- 
David Alan Black                      | Register for RubyConf 2002!
home: dblack@candle.superlink.net     | November 1-3
work: blackdav@shu.edu                | Seattle, WA, USA
Web:  http://pirate.shu.edu/~blackdav | http://www.rubyconf.com