From: Gary Wright <gwtmp01@...>
Date: 2007-03-02T08:06:59+09:00
Subject: Re: Newbie Question


On Mar 1, 2007, at 3:20 PM, james.d.masters@gmail.com wrote:
> On Mar 1, 11:35 am, Gary Wright <gwtm...@mac.com> wrote:
>> ...
>> I think you are asking for something which is quite easy:
>>
>> class A
>>    class <<self
>>      attr_accessor :per_class
>>    end
>> end
>> ...
>
> Yes, this is kind of what I'm referring to; however, my gripe is that
> this is somewhat esoteric.  The beauty of other parts of Ruby is that
> the code is terse and readable.  Also, I think that the work-around
> that you mentioned (and I've used too) does not truly inherit:

I think your use of 'inherit' is misleading.  Inheritance generally
refers to methods and not to data.  I think 'sharing' would be a better
word for what you are describing.

So you want to share data between a class and all its subclasses:

class A
   class <<self
     def a_tree
       @@a_tree
     end
     def a_tree=(val)
       @@a_tree = val
     end
   end
end

A.a_tree = 1

class B < A; end

puts B.a_tree    # 1

B.a_tree = 2

class C < A; end

puts C.a_tree            # 2
puts A.a_tree            # 2
puts A.new.class.a_tree  # 2
puts C.new.class.a_tree  # 2

Personally I'd avoid this sort of thing if at all possible. If you
are going to use class variables I think it is best to wrap the  
access to
them in methods.  It is often difficult to understand the lexical scope
of '@@var' and so if you only use the '@@' syntax when writing your
setters and getters (and thus limit yourself to a single lexical  
scope), you'll
be able to reason about how the data is shared more easily.  It also  
forces
all access to the shared state through well defined setters/getters,  
otherwise
access to class variables is almost as bad as using global variables.

Of course you don't even need class variables to do this:

class A
   class <<self
     a_tree = nil
     define_method(:a_tree) { a_tree }
     define_method(:a_tree=) { |val| a_tree = val }
   end
end

This works by forcing the setter and getters to share the
scope generated by the class<<self;end block, which is uniquely
associated with the class object A and uses a local variable,
a_tree, defined within that scope to reference the shared data.

Gary Wright