From: Brian Candler <B.Candler@...>
Date: 2007-05-20T21:34:58+09:00
Subject: Re: Why was the "Symbol is a String"-idea dropped?

On Sun, May 20, 2007 at 10:18:22AM +0900, Rick DeNatale wrote:
> On 5/19/07, Brian Candler <B.Candler@pobox.com> wrote:
> >On Sat, May 19, 2007 at 08:28:24AM +0900, Rick DeNatale wrote:
> 
> >> Ruby symbols are represented by a value computed by shifting the
> >> symbols integer representation left 8 bits and setting the low-order
> >> byte to 0xFF representation
> >
> >Perhaps it varies based on the Ruby version you're running; it's not like
> >that for me.
> >
> >irb(main):006:0> :foo.object_id.to_s(16)
> >=> "39490e"
> >irb(main):007:0> RUBY_VERSION
> >=> "1.8.4"
> 
> You can't really see the internal bit representations from ruby, since
> they get manipulated before you see them.  Much like the class of an
> object reported by ruby isn't the same as the object pointed to by its
> klass pointer at the C level.
> 
> And even if you could, I was talking about the integer representation
> of the symbol, not the object_id.

AFAIK, the object_id is the in-memory pointer to the structure of the object
(if it's a material object), or is one of the special values:

- 0, 2 or 4 for false, true or nil

- (n<<1) | 1 for Fixnums

None of these is valid as a pointer to a memory location, so they can be
recognised immediately as special.

So in the above, :foo's object ID looks like a memory pointer to me. It
might not be, but then you'd need to guarantee that 39490e could not
possibly be a valid memory pointer for some regular object (and also be able
to recognise this by inspection, i.e. by looking at the bit pattern)

Regards,

Brian.