From: Daniel Brockman <daniel@...>
Date: 2005-07-02T11:41:22+09:00
Subject: Re: How and where Fixnum are created

"Eust叩quio Rangel de Oliveira Jr." <eustaquiorangel@yahoo.com> writes:

> Hey Daniel. :-)
>
> | 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.
>
> A regular method is called on the object at a memory location
> allocated for the object value OR for value and a representation of
> the object class, with methods and so on?
>
> What is pointed by, let's say, a variable who stores a reference to
> a String like "this is a long string where I can write a lot of
> things" is just the string value or a complete package with it's
> value and methods?

Basically just the string data and a pointer to the String class.

> What will be released by the garbage collector is the answer of the
> question above (memory allocated for values OR for values and
> methods), when is not needed anymore, right?

Just the memory for the data.  The methods are still needed because
there are other strings around.

> For example on
>
> s1, s2, s3, s4 = "this","is","a","test"
>
> s1, s2, s3, s4 shares the String methods and their references points
> to memory locations only with "this" "is" "a" "test", querying the
> String methods on a String representation

Well, a String object is not just a character array,

   struct RBasic {
       unsigned long flags;
       VALUE klass;
   };

   struct RString {
       struct RBasic basic;
       long len;
       char *ptr;
       union {
           long capa;
           VALUE shared;
       } aux;
   };

but on the other hand, the list of method pointers is certainly not
duplicated for each and every string out there.

[Good call showing actual source code, Dominik.]

> OR
>
> the reference to "this" & object methods, "is" & object methods, "a"
> & object methods and "test" & object methods?

No, this is not how it works.  That would be terribly inefficient.


[...]

> I think it should be first one, specially when we change the String
> class at runtime and it changes all the String references, but I'm
> asking just to be sure. :-)

You are correct. :-)

> |    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.
> 
> That give us the value of the Fixnum, where we can find it's real
> value, as you explained, and on this case the method (snufkin) is
> looked up on a generic (or "internal") Fixnum class, like I supposed
> String above, not on the reference (the reference is the own object)
> as above right?  Kind of
> 
> n1, n2 = 1, 2
> 
>       +-------+   +---------+   +-------+
>       | value |   | methods |   | value |
>       +-------+   +---------+   +-------+
>  n1 = | 1     +---| succ    +---+ 2     | = n2
>       +-------+   +---------+   +-------+

No, what happens is more like this:

                      +---------+
   n1 = 0x00000003    | Fixnum  |
                      +---------+
   n2 = 0x00000005    | succ    |
                      +---------+

The `n1' and `n2' variables don't reference any _actual_ objects.
Ruby pretends the objects are there so we can feel warm and fuzzy.
It would be too inefficient to allocate a separate memory block for
each and every small integer out there.

> So we can make math right with it's value, as Robert said, not
> needing to make an overhead with methods and so on, right?  Robert
> answered me about this but his email is on my mailbox on my job. :-(

Right.  Converting plain C integer to mangled Ruby fixnum (INT2FIX) or
the other way around (FIX2INT) is really cheap.

> | Okay, I realize that this explanation turned out pretty confusing.
> | For more information, you might search for ``ruby immediate values.''
> 
> No, I really appreciated it. Thank you very much. Sorry for bore you
> guys with this kind of thing but I'm really curious. :-)

I'm glad I could be of help.  If this subject truly bored me, I would
probably not have responded to your question at all.  The truth is
that I needed to freshen up a bit on this myself. :-)

> I searched for that on the web, but I really needed some more
> "deeper" info, because there's a lot of places with just replicate
> the pickaxe contents. :-)

Ah, yes, that's annoying.  Man pages, in particular, tend to be
replicated across so many pages that they pollute search results.
One copy is quite enough please.

-- 
Daniel Brockman <daniel@brockman.se>

    So really, we all have to ask ourselves:
    Am I waiting for RMS to do this?   --TTN.