From: Joel VanderWerf <vjoel@...>
Date: 2006-09-13T13:49:00+09:00
Subject: Re: Wrapping a C Library: Allocation and duplication issues

Justin Bonnar wrote:
> I'm writing a wrapper for a C library [1] written in a very object 
> orientated style.  All of the "classes" are created with a 
> librdf_new_classname method which acts almost the same as Ruby's 
> Class#new.
> 
> The problem I'm having is cleanly separating allocation from 
> initialization.  Since most of the class construction functions require 
> arguments to initialize the object, I can't define separate alloc and 
> initialize functions.
> 
> The only option I see is to use Ruby 1.6 style code and define a 
> Klass.new method for my classes.  The problems with this is it prevents 
> clean subclassing and object cloning.  Is there a better way to wrap the 
> library?
> 
> Thanks,
> Justin
> 
> [1] http://librdf.org/
> 

What about calling librdf_new_classname() from the initialize method, 
and not doing anything special in the alloc function?

README.EXT says:
> To define and undefine the `allocate' class method,
> 
>   void rb_define_alloc_func(VALUE klass, VALUE (*func)(VALUE klass));
>   void rb_undef_alloc_func(VALUE klass);
> 
> func have to take the klass as the argument and return a newly
> allocated instance.  This instance should be empty as possible,
> without any expensive (including external) resources.

The ary_alloc function for Array, for example, does not allocate storage 
for the actual C array of VALUE. It only allocates the basic RArray 
object, with a null ptr. Then rb_ary_initialize() looks at the arguments 
and allocates storage for ptr, as needed.

-- 
       vjoel : Joel VanderWerf : path berkeley edu : 510 665 3407