From: William Pietri <william-news-383910@...>
Date: 2002-08-30T01:33:01+09:00
Subject: Re: Newbie converting brain from perl


Thanks to all who gave advice about the best way to get matching regexp 
items out; I found it very helpful. After years of doing most of my OO work 
in Java, it's exciting to be reminded that an OO language can be as 
programmer-friendly as Perl.


Here's the next thing that I'm puzzled about.  I'm looking for the most 
efficient way to do a Factory pattern.

When parsing a system log file, there are lots of lines of different sorts. 
It would make sense that one should have a base class, LogLine, and then a 
bunch of subclasses for particular entries (SendmailLogLine, NamedLogLine, 
etc.).

So one might start out with

    class LogLine
        attr_reader :month, :day, :hour, :minute, :second, 
                :host, :program, :pid, :message
        [...]
    end


But from there, I have two questions:

1) Is it possible (or even sensible) to override LogLine.new so that it 
returns an instance of the appropriate subclass? I've never liked the 
Java-style LogLine.getInstance(args); it seems unfair to make programmers 
keep track of whether you're using a Factory internally or not.

2) What's the most efficient way to give each subclass a chance to say "Yes, 
that line is best represented by me," without doing a lot of repetitive 
parsing?

The obvious OO way seems to be that each subclass would, on loading, 
register itself with LogLine. As each new LogLine is created, the 
superclass would show the line to each of the subclasses, and whichever 
subclass liked it the best would be instantiated with the data from the 
line.

Done naively, this would mean that each subclass would parse the line, but 
that would be wasteful. So obviously, the superclass should do the basic 
parsing once and pass the details down. Passing as an Array is brittle and 
not very OO. Passing as a Hash is better, but still not right.

Really, the parsed data is best represented as a LogLine, no? So it would 
make sense to instantiate a LogLine with the base data, and then pass that 
to the subclasses. But when instantiating the subclass, the subclass has to 
copy over all the data and throw out the copy of the superclass, which 
seems a little wasteful.

So is it possible for a subclass to somehow replace an instance of its 
superclass with one of its own, stealing its data without explicitly 
copying each field? 

I suspect the answer to this is "no", but Ruby's OO is so much more flexible 
than what I'm used to that I thought it would be good to ask.


Many thanks,

William