From: John Maclean <info@...>
Date: 2006-01-21T09:11:11+09:00
Subject: Re: Basic Inheritance questions {noob alert:- pickaxe ed. 2,

I'm sorry I still don't get it. (To be honest I think that I can narrow
down this problem to the book not having all of the code in a single
place)...

I can understand that for example (p27) is telling us that there's a
"right way" and "wrong way" to specify methods for child instances.. so
what's wrong with this?

#!/usr/bin/env ruby 
require 'class_Song.rb'

# test if this class has been loaded correctly
Object.const_defined?(:Song)
    
class KaraokeSong < Song
  # grab all the properties from the parent but add another instance
variable
  def initialize(lyrics)
    @lyrics = lyrics
  end
    
  def to_s
    super + " [#@lyrics]"
  end
end

song = KaraokeSong.new("And now, the...")
#song = KaraokeSong.new("My Way", "Sinatra", 225, "And now, the...") 
song.inspect 
song.to_s 
puts song

The above has not specified a name, artist and duration - but that's
 coming from the parent class, is it not?


jayeola@tp20$ ruby -w class_KaraokeSong.rb
./class_Song.rb:12: warning: instance variable @name not initialized
./class_Song.rb:12: warning: instance variable @artist not initialized
./class_Song.rb:12: warning: instance variable @duration not initialized
./class_Song.rb:12: warning: instance variable @name not initialized
./class_Song.rb:12: warning: instance variable @artist not initialized
./class_Song.rb:12: warning: instance variable @duration not initialized
Song: -- () [And now, the...]



On Sat, 21 Jan 2006 07:37:03 +0900
Eero Saynatkari <ruby-forum-reg@mailinator.com> wrote:

> John Maclean wrote:
> > Thanks for another fast response. Inheritance - I'm going to get
> > this -tonight-!
> > 
> > recap;
> > # the parent class
> > cat fx/ruby/chap3/class_Song.rb
> > #!/usr/bin/env ruby
> > class Song
> >   def initialize(name, artist, duration)
> >     # define instance variables
> >     @name = name
> >     @artist = artist
> >     @duration = duration
> >   end
> > 
> >   def to_s
> >     # method for displaying these instancess
> >   "Song: #@name--#@artist (#@duration)"
> >   end
> > end
> > 
> > # the child class
> > 
> > jayeola@tp20$ cat fx/ruby/chap3/class_KaraokeSong.rb
> > #!/usr/bin/env ruby
> > require 'class_Song.rb'
> > 
> > # test if this class has been loaded correctly
> > Object.const_defined?(:Song)
> > 
> > class KaraokeSong < Song
> >   def initialize(name, artist, duration, lyrics)
> >     super(name, artist, duration)
> >     @lyrics = lyrics
> >   end
> > 
> >   def to_s
> >     super + " [#@lyrics]"
> >   end
> > end
> > 
> > #song = KaraokeSong.new("My Way", "Sinatra", 225, "And now, the...")
> > #song = KaraokeSong.new("Sinatra", 225, "And now, the...")
> > #song.to_s
> > #song.inspect
> > 
> > 
> > How do we get KaraokeSong to acquire it's parent's attributes
> > -without- hadrcoding "def initialize(name, artist, duration,
> > lyrics)", unless one has to? From the book I get the impression that
> > you can get away with specifying the child's stuff and that it will
> > take properties from it's parent with the keyword super....
> 
> If the child needs additional parameters (or otherwise overrides
> the corresponding method in the parent class), you must* define
> the method in the child (super will run the parent's method like
> in your #initialize above). When the parent's behaviour is adequate,
> you do not need to define a method, it is automatically inherited.
> 
> An alternative would be to just have an accessor for @lyrics; that
> way you would not need to define #initialize, but you would need to
> call #lyrics= to set the lyrics initially., which on the surface
> would seem the more cumbersome way.
> 
> 
> E
> 
> * Not strictly true; you could circumvent this with some careful
>   metaprogramming but it really is not worth it in this situation.
> 


-- 
John Maclean
MSc (DIC)
07739 171 531