From: Michael Neumann <neumann@...>
Date: 2001-07-21T19:46:50+09:00
Subject: [ruby-talk:18244] RCR: Transient instance variables

Hi,

Currently it is very time consuming to leave some instance varibles
out of marshalling. You have to overwrite _dump and _load and write 
an appropriate initialize method.
For me this is inacceptable. So why not let Marshal leave out instance
variables that are named in the class instance variables "@transient"?

class A
  @transient = :trans_var, :a

  attr_reader :trans_var, :a
  attr_accessor :b, :c
end

I guess doing this in marshal.c would be only a minor change (array subtraction?), 
whereas to do the same in pure Ruby is much harder to accomplish (but it's possible):

class Transient
  def _dump(depth)
    hash = {}
    trans = self.class.class_eval("@transient")
    trans = [trans] unless trans.kind_of? Array
    trans = trans.collect {|i| "@#{i}"}
    (self.instance_variables - trans).each {|var|
      hash[var] = self.instance_eval("#{var}")
    }
    Marshal.dump(hash, depth-1)
  end
 
  def self._load(str)
    hash = Marshal.load(str)

    Thread.critical = true
    self.module_eval %{
      begin
        alias __initialize initialize
      rescue NameError
      end
      def initialize; end
    }

    obj = self.new 

    self.module_eval %{
      undef initialize
      begin
        alias initialize __initialize
      rescue NameError
      end
    }
    Thread.critical = false

    hash.each {|var, val|
      obj.instance_eval("#{var} = val")
    }
    obj
  end
end


class B < Transient
  @transient = :a
  def initialize(a, b)
    @a, @b = a, b
  end
end

b = B.new("transient!!!", 2)
p b
c = Marshal.load(Marshal.dump(b))
p c


Regards,

  Michael


-- 
Michael Neumann
merlin.zwo InfoDesign GmbH
http://www.merlin-zwo.de