From: Jeff Coleman <progressions@...>
Date: 2006-04-08T15:18:00+09:00
Subject: Re: Simple Question About Deleting Instances

Nathan O. wrote:
> Austin Ziegler wrote:
>> No. To clarify my statement:
>> 
>>   #attr_accessor creates a pair of instance methods that access and
>>   may instantiate an instance variable, but don't have to.
>> 
>> Instance variables are always bound to a particular value of self. (I'm
>> not *quite* sure where class variables reside.)
>> 
> ...
>> 
>> "class Foo", "class << self", and "def myself" all change scope and the
>> meaning of "self" in that scope.
> Way, way over my head, but that probably has something to do with the
> two-year-old demanding that I stop reading and start wrestling with him.
> I'll sleep on this one. Thanks! I think I might have a shot at
> understanding this one in the morning.

As traditional as they are, for me the Foo/bar style examples can be 
confusing because they give no context.

The Pickaxe book gives a good example of the difference between instance 
variables and class variables and how they can be useful.  Perhaps this 
is stuff you already know, but it can be helpful to cover it again.

In the Pickaxe example (paraphrasing the code), they're making a Jukebox 
application which plays songs and records the total number of plays of 
each song, as well as the total number of songs played by the jukebox.

class Song
  attr_accessor :name, :artist, :duration, :plays

  @@total_plays = 0   # class variable, the total number of all songs 
played by the jukebox

  def initialize(name, duration)
    self.name = name               # using the accessor methods
    self.artist = artist           # using the accessor methods
    self.duration = duration       # using the accessor methods
    plays = 0                      # using the accessor methods
  end

  def play
    @@total_plays += 1    # increase the total count
    self.plays += 1       # increase the count for this song
    # play the song
  end
end

So if you do this:

so_what = Song.new("So What", "Miles Davis", 9.24)

so_what.play
so_what.play
Song.total_plays   #=>  2
so_what.plays      #=>  2

penny_lane = Song.new("Penny Lane", "The Beatles", 3.45)

penny_lane.play
penny_lane.play

Song.total_plays   #=> 4
penny_lane.plays   #=> 2

So the "@@total_plays" variable stores a value unique to the class of 
Song, regardless of instances.  And the instance variables store values 
unique to each instance.  The class variable can be "declared" or used 
in the body of the class definition, but the instance variable should be 
used in the body of a method.

I'm sure you know most of this, but hopefully it helps a bit.

Jeff Coleman

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