Basic Inheritance questions {noob alert:- pickaxe ed. 2, page 27}

Discussion in 'Ruby' started by John Maclean, Jan 20, 2006.

  1. John Maclean

    John Maclean Guest

    Chaps,

    Say you have a class Song. You'd make a file for that class with the
    following;

    #!/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

    # test the class by creating a new object
    #song = Song.new("Bicylops", "Fleck", 260)
    #song.inspect

    Now it's time to make a "child" class....

    jayeola@tp20$ cat class_KaraokeSong.rb
    #!/usr/bin/env ruby
    class KaraokeSong < Song
    def initialize(name, artist, duration, lyrics)
    super(name, artist, duration)
    @lyrics = lyrics
    end

    # def to_s
    # # method for displaying these instancess
    # "Song: #@name--#@artist (#@duration)"
    # end
    end

    When I created the parent it was simple to `irb` and then `require
    class_Song.rb`.

    How would one use this child class? Not quite clear from the Pickaxe
    book. Do I create a new file for KaraokeSong as I have done or have
    both the "super" and "child" in the same file?
     
    John Maclean, Jan 20, 2006
    #1
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  2. You can do it one of three ways:
    In IRB, require the Song file, and then the KaraokeSong file, in that order=
     
    Wilson Bilkovich, Jan 20, 2006
    #2
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  3. John Maclean

    John Maclean Guest

    Thanks for that speedy reply. I've decided to require the within the
    child. One thing to note is that when I do require the child irb shows
    the following..

    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 to_s
    super + " [#@lyrics]"
    end
    end

    irb(main):011:0> require 'class_KaraokeSong.rb'
    => false
     
    John Maclean, Jan 20, 2006
    #3
  4. Getting a 'false' in irb means the file is already
    loaded (a LoadError would be raised if the file could
    not be found); if you want, you can use

    load 'song.rb'

    This will reload the file each time (seldomly necessary).

    As a compatibility-note, you could make your filenames
    all lowercase_and_underscored. This will thwart any
    case sensitivity issues.

    E
     
    Eero Saynatkari, Jan 20, 2006
    #4
  5. John Maclean

    John Maclean Guest

    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....
     
    John Maclean, Jan 20, 2006
    #5
  6. 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.
     
    Eero Saynatkari, Jan 20, 2006
    #6
  7. John Maclean

    John Maclean Guest

    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...]



     
    John Maclean, Jan 21, 2006
    #7
  8. John Maclean

    Mike Stok Guest

    ... shouldn't you have a call like

    super('','','')

    here to initialise the parent class variables?
    --

    Mike Stok <>
    http://www.stok.co.uk/~mike/

    The "`Stok' disclaimers" apply.
     
    Mike Stok, Jan 21, 2006
    #8
  9. John Maclean

    Mike Stok Guest

    ... shouldn't you have a call like

    super('','','')

    here to initialise the parent class variables?
    --

    Mike Stok <>
    http://www.stok.co.uk/~mike/

    The "`Stok' disclaimers" apply.
     
    Mike Stok, Jan 21, 2006
    #9
  10. There is no #initialize method that takes four parameters
    at this point (see below).
    All you are getting from the parent class are the actual
    variables and an #initialize method that takes three parameters.
    In your child class, you define a new #initialize that only takes
    one parameter (overriding the one from the parent class) so that
    all KaraokeSong.new can see is the one with just one parameter.

    Even if the child class had a method looking like this:

    def initialize(lyrics)
    super
    @lyrics = lyrics
    end

    The problem would still be that the method itself does not
    take more than one parameter and you would see an error if
    you tried to pass it four. So the correct way indeed is to
    have your child class with an #initialize that takes all
    four parameters and then call #super with the three parameters
    that must go to the parent- (or super-) class.

    E
     
    Eero Saynatkari, Jan 21, 2006
    #10
  11. John Maclean

    John Maclean Guest

    Thanks. The child is behaving at last ;)

    I think that I'll be grabbing a new book once I find one with slightly
    clearer descriptions and code base.

    Now I understand that one must declare (if that's the right word), all
    of the instance variables in for a child class and any variables that
    it will inherit from it's parent. Do the same for the child's method.


     
    John Maclean, Jan 21, 2006
    #11
  12. The pickaxe is as far as I know the best book available on the market.
    the pickaxe very clear and concise. I have since accomplished at least
    ten times as much with ruby as I ever have with any other language.

    It sounds like this is simply an issue of needing to experiment and
    learn from mistakes to build a better fundamental understanding of
    common structures. The best way to learn is by doing, as is true for
    all fields. I would just fling yourself in, attempt some of the ruby
    quiz challenges and have a good time. Whenever you get stuck or have a
    problem, the community here is very friendly and helpful.
     
    Timothy Goddard, Jan 21, 2006
    #12
  13. This test is completely superfluous. First, you don't do anything with
    the return value and second the next line will throw anyway:
    This just indicates that the file has been required already.

    Kind regards

    robert
     
    Robert Klemme, Jan 21, 2006
    #13
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