From: Gavin Sinclair <gsinclair@...>
Date: 2002-10-13T09:15:01+09:00
Subject: Re: Things That Newcomers to Ruby Should Know


Interesting; thanks!

Gavin

From: "William Djaja Tjokroaminata" <billtj@y.glue.umd.edu>


> Hi Gavin,
>
> That's because 1000000001 still can fit in Fixnum, so Ruby calls INT2FIX
> which simply left-shift by one and set the least significant bit to
> 1.  When you call "id", unless the object is a special constant, Ruby
> simply set the least significant bit to 1, and return the "VALUE" of the
> object (which for a Fixnum is the object itself).  Because the least
> significant bit is always set to 1 (unless probably for special
> constants), you will always see odd numbers for id's.
>
> So for all objects of type Fixnum, its id is simply given by left shift by
> one and set least significant bit to 1, which is simply equivalent to
> multiply by two and add 1.
>
> When you call "1000000001.id.class", the "class" method of
> "1000000001.id" is called; because in this case 2000000003 does not
> fit into a Fixnum, Ruby creates a Bignum, and returns its class.
>
> So the id's are not predefined (not stored in memory), but created
> (computed) as necessary based on the simple formula above.
>
> Finally, when you ran the successive testlets and you found many more
> objects, I guess it is just because the testlet itself creates many more
> Ruby (temporary) objects, which exist until the next gc cycle.
>
> Regards,
>
> Bill
> ============================================================================
> Gavin Sinclair <gsinclair@soyabean.com.au> wrote:
> > When I discovered this some time ago, I wondered deeply what might be
occupying
> > id #s 2, 4, 6, ...
>
> > Interesting that "id" is documented in 'ri' to return a Fixnum, but:
> >   1000000001.id              => 2000000003
> >   1000000001.id.class        => Bignum
>
> > I'm surprised that numbers that high have a predictable ID, as if they're
> > predefined.  But then maybe the numbers are brought into being only as
needed
> > (except for the first 100, like Python), with a special case formulaic
approach
> > to assigning them an ID, and then all non-Integers are given an even ID so
they
> > don't clash.  When I ran
>
> >    a = []
> >    ObjectSpace.each_object { |obj| a << obj }
> >    a.find { |obj| obj.id % 2 == 1 }
>
> > it returned nil, so that must be the case.
>
> > What I also found interesting was that successive runs of this testlet
seemed
> > to produce many more objects.
>
> > Any comments?
>
> > Gavin
>
>
>