From: Robert Klemme <shortcutter@...>
Date: 2008-10-17T18:29:32+09:00
Subject: Re: instance counter

On 17.10.2008 07:19, Mike Gold wrote:
> Tommy Nordgren wrote:
>> def Bob.count
>>   return @@count
>> end
> 
> I avoid @@.  Why should metaclasses have a different syntax?  @@ also 
> has different scoping rules which I find vexing.  Be consistent, I say.
> 
> class Bob
>   @count = 0
> 
>   class << self
>     attr_reader :count
>   end
> 
>   def initialize
>     Bob.instance_eval { @count += 1 }
>   end
> end
> 
> a = Bob.new
> b = Bob.new
> puts Bob.count # => 2
> 
> And in general,
> 
> def new_instance_counted_class
>   Class.new {
>     count = 0
> 
>     (class << self ; self ; end).instance_eval {
>       define_method(:count) {
>         count
>       }
>     }
> 
>     define_method(:initialize) {
>       count += 1
>     }
>   }
> end
> 
> class Bob < new_instance_counted_class
>   # ...
> end
> 
> a = Bob.new
> b = Bob.new
> puts Bob.count # => 2
> 
> To those who were asking about Class.new{} from one of my previous 
> posts, here is a good example.
> 
> The local variable "count" lives in the closure, and is private in the 
> ultimate sense.  There are no scoping rules to worry about, no efforts 
> are needed to choose a @hopefully_unused_variable_name, no surprising 
> subclass behavior, and no possibility that an instance_eval can rape the 
> count.
> 
> Try writing the above using the conventional Java-like "blub" 
> constructs.  You'll have some or all the problems I just mentioned.

I nevertheless believe there is a more Rubyish solution to achieve what 
you did above with new_instance_counted_class:

module InstanceCounted
   module InstanceCounter
     attr_accessor :count
   end

   def initialize(*a,&b)
     super
     self.class.count += 1
   end

   def self.included(cl)
     cl.extend InstanceCounter
     cl.count = 0
   end
end

class Foo
   include InstanceCounted
end

Note though that this approach (as yours above) has some shortcomings: 
the count is not adjusted when objects are garbage collected.  And you 
must invoke super if you define Foo#initialize (a good habit anyway) - 
in your case you need to alias the old initialize and call that if you 
redefine it.

There is another option though: you can get the current count via 
ObjectSpace:

count = 0
ObjectSpace.each_object(Foo) { c += 1 }

Kind regards

	robert