From: John Cooper <lists@...>
Date: 2012-05-16T23:51:38+09:00
Subject: Passing by reference - modifying instance variables

Hi all,

I'm new to Ruby and I've been working through the book "Programming
Ruby: The Pragmatic Programmer's Guide"

Working through their jukebox example, I have come across a couple of
things that don't quite make sense to me.

First, if I have a class with accessor methods but no mutator methods, I
can still modify the instance variables. The code in the attachment
reference.rb has the output:

Blur - Song 2 (180)
Blur - song 2 (180)

The instance variable title of song2 has been modified. This makes sense
to me, since the variables are passed by reference - I guess I'd need to
return a copy from my accessor method to avoid this happening. Is there
a neat way to do this?

To add further confusion, the code in the attachment reference2.rb -
with an identical definition of the class Song, with a call to
.downcase! in WordIndex::index does not modify the instance variables in
the Song class, even though, to my eyes, the title and artist strings
are being passed around just the same as in the previous simpler
example.

Attachments:
http://www.ruby-forum.com/attachment/7397/reference.rb
http://www.ruby-forum.com/attachment/7398/reference2.rb


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