From: Joel VanderWerf <vjoel@...>
Date: 2006-10-26T14:49:25+09:00
Subject: Re: Popularity Contest: Initializing Class-Level Instance Variables

Joel VanderWerf wrote:
> Phrogz wrote:
>> TECHNIQUE 2 - ON-DEMAND TEST/INITIALIZATION
>> -------------------------------------------
>> class Alpha
>>   def self.m1; (@foo||=[]) << rand; end
>>   def self.m2; (@foo||=[]).map{ rand }; end
>> end
>>
>> class Bravo < Alpha
>>   # Nothing special here
>> end
> 
> I prefer a variant of the above:

Oops, midway through that exercise I forgot we were talking about class 
methods, not instance methods. The following runs correctly:

require 'set'

class Alpha
   def self.foo
     @foo ||= []
   end
   def self.m1; foo << rand; end
   def self.m2; foo.map{ rand }; end
   # note that the expression for contstructing foo is not
   #  repeated twice
end

class Bravo < Alpha
   # Nothing special here
end

class Charlie < Alpha
   def self.foo
     @foo ||= Set.new # or some other collection class
   end
   # This implies that all _subclasses_ of Charlie will
   # (unless overridden) use a set for the #foo method
end

class Delta < Charlie
   # default to Charlie's choice of Set as the foo collection
end

class Echo < Alpha
   @foo = [1,2,3] # possible to specify in advance
end

[Alpha, Bravo, Charlie, Delta, Echo].each {|c| p c.m1}

[0.657784608794096]
[0.340975041442558]
#<Set: {0.191071815451883}>
#<Set: {0.971105015466828}>
[1, 2, 3, 0.58602201527139]

-- 
       vjoel : Joel VanderWerf : path berkeley edu : 510 665 3407