From: Ross Bamford <rossrt@...>
Date: 2006-10-31T03:09:35+09:00
Subject: Re: Alias singleton method from C

On Tue, 2006-10-31 at 02:10 +0900, Daniel Berger wrote:
> Tim Pease wrote:
> > On 10/30/06, Daniel Berger <djberg96@gmail.com> wrote:
> > >
> > > Good question.  I'm not sure why this code fails:
> > >
> > > #include <ruby.h>
> > >
> > > static VALUE foo_bar(){
> > >    return rb_str_new2("hello");
> > > }
> > >
> > > void Init_foo(){
> > >    VALUE cFoo, singleton;
> > >
> > >    cFoo = rb_define_class("Foo", rb_cObject);
> > >
> > >    rb_define_singleton_method(cFoo, "bar", foo_bar, 0);
> > >
> > >    singleton = rb_const_get(rb_cObject, rb_intern("Foo"));
> > >    rb_define_alias(singleton, "baz", "bar");
> > > }
> > >
> > > When I compile and run that it fails with "undefined method `bar' for
> > > class `Foo'" even though it clearly is defined.  I can see in class.c
> > > that rb_define_alias is just calling rb_alias from eval.c.  It looks
> > > like rb_alias handles singletons differently, but why it's not working
> > > in the example I provided I'm not sure.
> > >
> >
> > rb_define_alias only works for instance methods.
> 
> It should work for either singletons or instance methods.  If it
> didn't, then this code wouldn't work:
> 
> class Foo
>    class << self
>       def bar
>          "hello"
>       end
>       alias baz bar
>    end
> end
> 
> p Foo.bar => "hello"
> p Foo.baz => "hello"
> 
> The C example I pasted is meant to do what I've done there.
> 
> Internally rb_define_alias calls rb_alias (in eval.c) which checks to
> see if klass is a singleton or not.  So, in theory, I should be able to
> do:
> 
> rb_define_alias(singleton, baz, bar);
> 
> But obviously I'm doing something wrong - I'm just not sure what.

This is similar to what I was going to do - you just need to get the
singleton class from the class. I _think_ in your example, cFoo ==
singleton. Something like this works:

  /* define singleton methods as normal */

  VALUE singleton = rb_singleton_class(cXMLNode);
  rb_define_alias(singleton, "new_element", "new");
  
I guess that would work with rb_alias too, but I thought there had to be
a wrapper for this somewhere that I'd missed.

> PS - A rb_define_singleton_alias() would be a handy function. :)
> 

Agreed. Assuming it's not already lurking somewhere, that is. :)

Cheers,
-- 
Ross Bamford - rosco@roscopeco.REMOVE.co.uk