From: Robert Klemme <shortcutter@...>
Date: 2012-05-17T00:04:44+09:00
Subject: Re: Passing by reference - modifying instance variables

On Wed, May 16, 2012 at 4:51 PM, John Cooper <lists@ruby-forum.com> wrote:
> 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?

Dunno whether you find that neat

def foo
  @foo.dup
  # ultra save: @foo.dup rescue @dup
end

You could even create a custom variant of attr_reader which generates this.

Or you freeze instance variables.

> 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.

word.downcase! works on the scan result which is not the original string.

irb(main):001:0> s = "FOO"
=> "FOO"
irb(main):002:0> s.scan(/../) {|m| p m; m.downcase!; p m}
"FO"
"fo"
=> "FOO"
irb(main):003:0> s
=> "FOO"

Kind regards

robert


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