From: Robert Klemme <bob.news@...>
Date: 2005-01-01T03:46:41+09:00
Subject: Re: sizeof(Class)/sizeof(Object)


"Charles Mills" <cmills@freeshell.org> schrieb im Newsbeitrag 
news:1104515965.487836.265250@c13g2000cwb.googlegroups.com...
> Nikolai Weibull wrote:
>> I would like to figure out how much memory a certain class or object
>> will/is consuming.  Is there an easy way?  Is there a hard way?
>> The reason I ask is because I want to keep several hundred thousand
>> instances of an object much like the following one:
>>
>> class A
>> def initialize
>> @a = @b = @c = @d = nil
>> @e = 1
>> end
>> end
>>
>
> Instance variables are stored in a hash table associated with each
> object.  See rb_ivar_set() in variable.c.  So basically each instance
> will take up the size of struct RObject - which is 20 bytes on a 32bit
> machine, plus the size of the hash table (see st.h), plus something
> else I am probably missing.  As far as I know st.h doesn't provide any
> means of calcing the number of bytes used by the hash.
> Is there a way to do this?
>
> Also, objects with no instance variables don't create a hash table to
> store instance variables...

So if there was only one instance var an optimization could be to store them 
in a hash in the instance's class.  This could work for multiple ivars as 
well but then there is the array overhead.  Hm...

class Foo
  @ivar = {}

  def initialize
    cl = self.class
    ObjectSpace.define_finalizer(self) {|id| cl.remove id}
  end

  def bar=(x) self.class.set_val(id, x) end
  def bar; self.class.get_val(id) end

  def self.set_val(id, val)
    @ivar[id] = val
  end

  def self.get_val(id)
    @ivar[id]
  end

  def self.remove(id)
    puts "removing #{id}"
    @ivar.delete id
  end
end

Just some thoughts in code...

    robert