From: ptkwt@...1.aracnet.com (Phil Tomson)
Date: 2002-02-02T10:55:20+09:00
Subject: Re: Newbie question

In article <ad2f1094.0202011555.2887f94d@posting.google.com>,
andrew queisser <andrew_queisser@hp.com> wrote:
>Oliver Mensinger <cyo@gmx.net> wrote in message
>news:<Pine.LNX.4.21.0201301341560.312-100000@bogus.trumper>...
>Thanks, that almost gets me there. I wasn't clear enough in my
>original post but the "words" could consist of any characters other
>than comma. They include spaces, dashes, etc. I'm sure the regexp
>could be modified to account for that.
>
>Your post brings up another question: I noticed you're adding a
>function to the String class. My first thought was (my primary
>language is C++) "shouldn't we derive a new class and add the function
>there." Then I realized that I have a loop in my program that does a
>File.each_line and the extended string class dovetails nicely into the
>loop. I just call your new function on the block parameter (which is a
>String) like so:
>
>  infile.each_line do |line|
>    ofile.puts line.getCommaElements(4)
>  end
>
>Very nice. Then I started thinking about extending well-known classes
>in general and I'm wondering whether injecting new functions is such a
>good idea. After all, I could change my loop to this:
>
>  infile.each_line do |line|
>    ofile.puts CommaElementSearcher(line, 4)
>  end
>
>where CommaElementSearcher is a new function (or class) that handles
>the same functionality.
>
>So, what are the feelings about cost/benefit of modifying existing
>classes in this way? Are there hidden dangers or is it a good thing?

I think it tends to be a cultural thing.  Apparently in SmallTalk this is 
also a common practice.  I can see why it would be problematic in the C++ 
world because it really isn't as easy to do in C++ (actually, is it even 
possible in C++ at all to modify an existing class and have all current 
objects of that class respond accordingly? - I doubt it).

You probably should use caution in changing the way an existing method 
of a class works, though.  For example, it probably wouldn't be a good 
idea to change Array#each to do something different than it currently 
does.  But adding methods to a built-in class, as in your example, is 
fairly common practice in the Ruby community.

Phil