From: timsuth@... (Tim Sutherland)
Date: 2005-01-11T09:06:20+09:00
Subject: Re: Immediate values

In article <20050110211912.GA75928@topi.cc>, why the lucky stiff wrote:
[...]
>Hi, Ruth.  So, when someone on the list uses uppercase VALUE, they are
>refering to an object's symbol table id.  The symbol table being the
>global dictionary that pairs up a unique id with pointers to the actual
>objects.  VALUE is a C typedef.
[...]

There is no such global dictionary, all the information is encoded in the
VALUE.

See http://www.rubygarden.org/ruby?GCAndExtensions:

  Ruby objects are represented by VALUE in the Ruby C API. This is a typedef
  for unsigned long. The value 0 is used for false and 4 is used for nil.
  Otherwise, if the two least significant bits are 00 it is a pointer to a
  structure on the heap which represents the Ruby object. (i.e. if "value &
  0x03 == 0.") Other values are immediate objects or special constants (small
  integers, symbols, true). This works because the structures are always
  quad-aligned in memory. (So the pointer is always a multiple of 4, i.e.
  has lower bits 00.) Then for example FIX2INT converts a Fixnum to a C int
  by right-shifting the VALUE.