From: "Eustaquio Rangel de Oliveira Jr." <eustaquiorangel@...>
Date: 2004-12-30T22:17:17+09:00
Subject: Inheritance of class variables

Hello there.

I was talking with chris2 and matju on the irc channel and they gave me 
good explanation about this point, but I'd like to share it with you and 
discuss something about it.

I was asking about if, when defined on a parent class, a class variable 
overwrites the class variables with the same name on it's children.

For example:

------------------------------------------------------------------------
class CarBuilder
    @@total_of_cars=0
    def initialize(name)
       @name = name
    end	
    def build
       puts "#{@name} building another car ..."
       @@total_of_cars +=1
    end
    def total_of_cars
       "#{@name} built #{@@total_of_cars} cars"
    end
end

class Honda < CarBuilder
    @@total_of_cars=0
    def initialize
       super("Honda")
    end
end

class Ford < CarBuilder
    @@total_of_cars=0
    def initialize
       super("Ford")
    end
end

h = Honda::new
f = Ford::new

puts h.total_of_cars
puts f.total_of_cars

h.build
h.build
h.build
f.build

puts h.total_of_cars
puts f.total_of_cars
------------------------------------------------------------------------

Running this code we have:

------------------------------------------------------------------------
Honda built 0 cars
Ford built 0 cars
Honda building another car ...
Honda building another car ...
Honda building another car ...
Ford building another car ...
Honda built 4 cars
Ford built 4 cars
------------------------------------------------------------------------

So, there is a mistake here. Honda built 3 cars and Ford 1, and not 4 for 
Honda and 4 for Ford. They are sharing the parent class variable, not 
matter if it was defined on the children also.

Matsu told me that who came first is the boss, so if @@total_of_cars was 
defined on the parent, not matter what, all the other children will refer 
to it. I can live with that making some hacks ehehe but let me see:

------------------------------------------------------------------------
class CarBuilder
    def initialize(name)
       @name = name
    end	
    def build
       puts "#{@name} building another car ..."
       increase
    end
    def increase
    end
end

class Honda < CarBuilder
    @@total_of_cars=0
    def initialize
       super("Honda")
    end
    def increase
       @@total_of_cars +=1
    end
    def total_of_cars
       "#{@name} built #{@@total_of_cars} cars"
    end
end

class Ford < CarBuilder
    @@total_of_cars=0
    def initialize
       super("Ford")
    end
    def increase
       @@total_of_cars +=1
    end
    def total_of_cars
       "#{@name} built #{@@total_of_cars} cars"
    end
end

h = Honda::new
f = Ford::new

puts h.total_of_cars
puts f.total_of_cars

h.build
h.build
h.build
f.build

puts h.total_of_cars
puts f.total_of_cars
------------------------------------------------------------------------

Running it I got:

------------------------------------------------------------------------
Honda built 0 cars
Ford built 0 cars
Honda building another car ...
Honda building another car ...
Honda building another car ...
Ford building another car ...
Honda built 3 cars
Ford built 1 cars
------------------------------------------------------------------------

That is the correct behaviour.
But let check what I changed there on the code:

- I removed @@total_of_cars from CarBuilder, and put it on *each* child 
class (there was one row of code, now there is two - or more, if more 
children), and could be a problem if I miss the point on some child class 
and forget to declare that variable there).

- On the CarBuilder, I create an empty method there called "increase" just 
to call it on the "build" method (that I want the children inherit it's 
funcionality). Two more rows there.

- I also removed the total_of_cars method from CarBuilder, but needed to 
write it on each children. 3 rows less on the parent, but 3 more on each child.

- Also on the children I needed to write the increase method, 3 more rows 
on each child.

Sorry if I'm missing some point here, because I'm a Ruby newbie, but is 
that correct? I mean, seems that we need to write more code with this kind 
of behaviour.

And if we forget to implement some of the needed methods or the 
@@total_of_cars on each CarBuilder child (CarBuilder could have a lot of 
other methods that I could want to inherit also!), it could not work normally.

On another point, if all this methods and variables could be inherited from 
the parent class, will be no problem.

I don't really know if there is a strong and well-defined OOP concept on 
the fact that a child cannot overwrite it's parent class variables, but 
could you

- Tell me if there is some kind of OOP concept like that and
- Tell me if the way I made the code, even with more lines, is the correct 
and more efficient way to control how many cars each CarBuilder child made?

Thanks for your attention. :-)

----------------------------
Eust�quio "TaQ" Rangel
eustaquiorangel@yahoo.com
http://beam.to/taq
Usu�rio GNU/Linux no. 224050