From: Daniel Brockman <daniel@...>
Date: 2005-07-02T09:32:38+09:00
Subject: Re: How and where Fixnum are created

Eustaquio,

All Ruby objects are aligned on four-byte boundaries in memory, so all
pointers are divisible by four.  The rest of the pointer values are
used to store immediate values such as fixnums.

(This is a pretty standard way to store immediate values in dynamic
languages that lack static typing.)

So when you say this,

   foo = Object.new

an object is created and its address (which will be divisible by four)
is stored in `foo'.  But when you say this,

   bar = 42

the integer 42 is doubled, increased by one, and the resulting value
is stored directly in `bar'.  That's how Ruby stores fixnums.

Later, when you say this,

   foo.moomin

Ruby sees that `foo' is divisible by four and so must be a pointer to
an object, and a regular method call is made.  But when you say this,

   bar.snufkin

it sees that `bar' contains 85, which is an odd integer --- not
divisible by four.  The oddness means that `bar' is an immediate
Fixnum, so Ruby converts the value to its real meaning by subtracting
one and halving it, giving 42.  Then the method `snufkin' is looked up
in the Fixnum class and invoked with `self' bound to 42.

Okay, I realize that this explanation turned out pretty confusing.
For more information, you might search for ``ruby immediate values.''

I hope this helps at least a bit,

-- 
Daniel Brockman <daniel@brockman.se>