From: Thufir <hawat.thufir@...>
Date: 2007-11-01T05:27:43+09:00
Subject: why, or when to, use a class method?

Here's the example:

Jukeboxes charge money for each song played, not by the minute. That
makes short songs more profitable than long ones. We may want to
prevent songs that take too long from being available on the SongList.
We could define a class method in SongList that checked to see if a
particular song exceeded the limit. We'll set this limit using a class
constant, which is simply a constant (remember constants? they start
with an uppercase letter) that is initialized in the class body.

class SongList
  MaxTime = 5*60           #  5 minutes
  def SongList.isTooLong(aSong)
    return aSong.duration > MaxTime
  end
end
song1 = Song.new("Bicylops", "Fleck", 260)
SongList.isTooLong(song1) 	» 	false
song2 = Song.new("The Calling", "Santana", 468)
SongList.isTooLong(song2) 	» 	true

<http://www.ruby-doc.org/docs/ProgrammingRuby/>

Because the definition is declared with "def
SongList.isTooLong(aSong)", this indicates it to be a class variable
for the SongList class, ok so far.  Why a class variable?  All
SongList objects will use the same method, but wouldn't they use the
same method even if it were an instance method?  Methods are declared
on the class, not the instance.


The class variable makes more sense to me, I can see the need better:

"For example, our jukebox may want to record how many times each
particular song has been played. This count would probably be an
instance variable of the Song object. When a song is played, the value
in the instance is incremented. But say we also want to know how many
songs have been played in total. We could do this by searching for all
the Song objects and adding up their counts, or we could risk
excommunication from the Church of Good Design and use a global
variable. Instead, we'll use a class variable.

class Song
  @@plays = 0
  def initialize(name, artist, duration)
    @name     = name
    @artist   = artist
    @duration = duration
    @plays    = 0
  end
  def play
    @plays += 1
    @@plays += 1
    "This  song: #@plays plays. Total #@@plays plays."
  end
end"

There is one @plays per instance, but only one @@plays which all Song
objects share; this I follow.

Why use a class method in the first example, though?




thanks,

Thufir