From: Rick DeNatale <rick.denatale@...>
Date: 2007-09-28T06:06:24+09:00
Subject: Re: object_id 1, 2, 3

On 9/27/07, SpringFlowers AutumnMoon <summercoolness@gmail.com> wrote:
> Eric Hodel wrote:
> >
> > You don't need to go quite so far.  OS X starts its heap at a low
> > memory address:
> >
> > $ ruby -ve "p ''.object_id"
> > ruby 1.8.6 (2007-09-23 patchlevel 5000) [powerpc-darwin8.10.0]
> > 1002180
> >
> > $ ruby -ve "p ''.object_id"
> > ruby 1.8.6 (2007-03-13 patchlevel 0) [i386-freebsd6]
> > 67351450
> >
> > Also, the way ruby's heap is constructed, addresses will count down
> > then jump up to the next chunk of ruby heap slots (provided that no
> > GC is done):
> >
> > $ ruby -e "p %w[a b].map { |o| o.object_id }"
> > [1002060, 1002050]
>
> so the object_id can be just the memory location of the object?  does
> Ruby keep a table to tell whether it is a memory location as opposed to
>
> irb(main):003:0> 1000000.object_id
> => 2000001
>
> which is not a memory location... in other words, when Ruby tries to
> fetch the object, sometimes it will dereference 1002060 (fetch the
> content there) but sometimes it won't, but just pretend that it is an
> actual object?
>
> I just realize that what if we extend the class Fixnum to have an extra
> instance variable
>
>
> class Fixnum
>   attr_accessor :foo
> end
>
> p 1.foo                # nil
>
> 1.foo = 123
> p 1.foo                # 123
> p 1.object_id          # 3
>
> so in this case, the object_id is not the memory location... and where
> is the memory location?  (for storing the instance variable)
>

For objects with immediate values, there is a global hash table which
maps the object_id to the location of the instance variables.

Note that "extend(ing) a class to have an extra instance variable" is
a bit misleading for Ruby.

Unlike many other OO languages instance variables in Ruby aren't
really associated with a class.
An instance acquires an instance variable not because it's a member of
a class, but because a method with that object as self was executed
which initialized it.

irb(main):001:0> class Fixnum
irb(main):002:1>   attr_accessor :foo
irb(main):003:1>   def foo_defined?
irb(main):004:2>     defined? @foo
irb(main):005:2>   end
irb(main):006:1>   end
=> nil
irb(main):007:0> 1.foo_defined?
=> nil
irb(main):008:0> 1.foo = 123
=> 123
irb(main):009:0> 1.foo_defined?
=> "instance-variable"
irb(main):010:0> 2.foo_defined?
=> nil

This is true of all objects, not just immediates.

irb(main):011:0> class Glorp
irb(main):012:1>   attr_accessor :bar
irb(main):013:1>   def bar_defined?
irb(main):014:2>     defined? @bar
irb(main):015:2>   end
irb(main):016:1> end
=> nil
irb(main):017:0> g = Glorp.new
=> #<Glorp:0x5ba68>
irb(main):018:0> g.bar_defined?
=> nil
irb(main):019:0> g.bar = :fred
=> :fred
irb(main):020:0> g.bar_defined?
=> "instance-variable"

-- 
Rick DeNatale

My blog on Ruby
http://talklikeaduck.denhaven2.com/