From: Robert Klemme <bob.news@...>
Date: 2005-01-02T19:11:38+09:00
Subject: Re: Inheritance of class variables


"darren" <rubylang@usmstudent.com> schrieb im Newsbeitrag 
news:200501011014.19101.rubylang@usmstudent.com...
>I think I understand all of the code in this thread (see below) except for 
>the
> 2nd line "attr_reader :name, :total_of_cars".  Could you, or someone, 
> explain
> this line to me, please?
>
> Also, I tried putting this code into a single *.rb file and I couldn't get 
> any
> output.  I did get a few warnings, however:
>
> ex.
> /1foo.rb:4: warning: parenthesize argument(s) for future version
> /1foo.rb:7: warning: parenthesize argument(s) for future version
> /1foo.rb:8: warning: parenthesize argument(s) for future version
> /1foo.rb:12: warning: parenthesize argument(s) for future version
> /1foo.rb:12: warning: parenthesize argument(s) for future version
> /1foo.rb:13: warning: parenthesize argument(s) for future version
> /1foo.rb:4: undefined method ` ' for CarBuilder:Class (NoMethodError)
>
> I guess the warnings mean that some syntax is being deprecated in later
> versions.  Assuming that is correct, I still don't see what is generating 
> the
> last error.

Which version of Ruby are you using?  I tested with 1.8.1 and didn't see any 
of those errors.  Maybe it's a copy and paste problem from the mail?  Line 
endings could be a reason.

> So, I tried using irb to enter it line by line.  However, when I get down 
> to
> the where I declare the first instance (h = CarBuilder.new "Honda"), irb
> gives me the error, "NameError: uninitialized constant CarBuilder".

Works for me:

$ irbs
>> class CarBuilder
>>   attr_reader :name, :total_of_cars
>>
?>   def initialize(name)
>>     @name = name
>>     @total_of_cars = 0
>>   end
>>
?>   def build
>>     puts "#{name} builds another car"
>>     @total_of_cars += 1
>>   end
>> end
=> nil
>>
?> h = CarBuilder.new "Honda"
=> #<CarBuilder:0x100ccce0 @name="Honda", @total_of_cars=0>

> Finally, I don't see where you are keeping up with the actual total of 
> cars
> built by all manufacturers.

My example code doesn't.

>  So, for practice, I was going to try to extend
> this code to handle that.  Would the proper way to do that be to declare a
> class variable (like @@collective_total = 0) and put it somewhere inside 
> of
> the class (like right after the attr_reader . . . line)?  Then, inside of 
> the
> build method, bump the collective_total?

I would not use a class var.  Rather use an instance var of the class.

You could also query all instances which reduces redundancy and thus error 
likelyness:

>> total = 0; ObjectSpace.each_object(CarBuilder) {|o| total += 
>> o.total_of_cars}; total
=> 0
>> h.build
Honda builds another car
=> 1
>> h.build
Honda builds another car
=> 2
>> h.build
Honda builds another car
=> 3
>> total = 0; ObjectSpace.each_object(CarBuilder) {|o| total += 
>> o.total_of_cars}; total
=> 3

Of course, if you need the number of  all cars often then it's more 
efficient to introduce redundancy and have a total of all car makers 
available.

Regards

    robert