Hi --

On Sun, 11 Oct 2009, dreamcat four wrote:

> Hi,
> I'm having trouble understanding why this isn't available from the Ruby core?
> Does core not provide an alternative / satisfactory implementation of this?
>
> # clia.rb - Class Level Inheritable Attributes
>
> module ClassLevelInheritableAttributes
>  def self.included(base)
>    base.extend(ClassMethods)
>  end
>
>  module ClassMethods
>    def inheritable_attributes(*args)
>      @inheritable_attributes ||= [:inheritable_attributes]
>      @inheritable_attributes += args
>      args.each do |arg|
>        class_eval %(
>          class << self; attr_accessor :#{arg} end
>        )
>      end
>      @inheritable_attributes
>    end
>
>    def inherited(subclass)
>      @inheritable_attributes.each do |inheritable_attribute|
>        instance_var = "@#{inheritable_attribute}"
>        subclass.instance_variable_set(instance_var,
> instance_variable_get(instance_var))
>      end
>    end
>  end
> end
>
> class Polygon
>  include ClassLevelInheritableAttributes
>  inheritable_attributes :sides
>  @sides = 8
> end
>
> puts Polygon.sides # => 8
>
> class Octogon < Polygon; end
>
> puts Polygon.sides # => 8
> puts Octogon.sides # => 8
>
> Ref: http://railstips.org/2006/11/18/class-and-instance-variables-in-ruby
>
> Its rather difficult to believe we must in each case define / require
> our own custom module. Please note - i'm not complaining here. I just
> want to know whether the above example is "the best solution in use"
> for 1.8... 1.9....

I would probably write it a little differently, maybe like this:

module ClassLevelInheritableAttributes
   def inheritable_attributes(*args)
     (class << self; self; end).class_eval do
       attr_accessor *args
     end

     @inheritable_attributes ||= []
     @inheritable_attributes.concat(args)
   end

   def inherited(subclass)
     @inheritable_attributes.each do |attr|
       subclass.send("#{attr}=", send(attr))
     end
     super
   end
end

class Whatever
   extend ClassLevelInheritableAttributes
end

etc. I'm not sure it's a common enough idiom to be worthy of a new
language construct, though. I love the fact that you can make what are
essentially language-level-like constructs from a few lines of code in
Ruby. I think that very, very few of them should be promoted to the
actual language.


David

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David A. Black/Ruby Power and Light, LLC (http://www.rubypal.com)