From: Mike Gold <mike.gold.4433@...>
Date: 2008-10-17T14:19:27+09:00
Subject: Re: instance counter

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.
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