From: Quintus <quintus@...>
Date: 2012-05-08T17:10:14+09:00
Subject: Re: How to use Data_Wrap_Struct to assign the DATA VALUE to an exsiting Ruby object?

Am 08.05.2012 00:21, schrieb I��aki Baz Castillo:
> Hi, my code receives an arbitrary klass name (provided by the user)
> and I need to create an instance of such a klass and also wrap a C
> pointer within it.
> 
> Unfortunately Data_Wrap_Struct(VALUE class, void (*mark)(), void
> (*free)(), void *ptr") requires a class VALUE rather than a single
> object/instance. So I expect that I *cannot* attach/wrap a C DATA
> pointer into an *existing* Ruby object and, instead, I need to store
> the DATA pointer within an internal attribute of the object, am I
> right?
> 
> Thanks a lot.
> 

No, it���s possible, but not documented. I���m not entirely sure how I did
this, but it involved reading the bare RDATA struct from the the object
and changing its internal pointer directly (maybe you should try hunting
around in ruby.h/intern.h for the exact struct definitions). My usecase
however was a bit different; I had to place a pointer already at the
class instanciation, and then replace it later with a pointer to
something different, i.e. my object was already wrapping something. I���m
not sure if the trick via RDATA does work with objects that haven���t been
assigned something right from the beginning.

Vale,
Marvin