From: John W Higgins <wishdev@...>
Date: 2009-06-16T03:53:37+09:00
Subject: Re: wrapping a C struct[] constant

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On Mon, Jun 15, 2009 at 11:11 AM, Martin DeMello <martindemello@gmail.com>wrote:

> On Mon, Jun 15, 2009 at 7:13 PM, Jason Roelofs<jameskilton@gmail.com>
> wrote:
> >
> > Could you give an example of how this would work? I'm having a hard time
> > picturing how such a thing would be possible to have done automagically.
> > There has to be something somewhere that defines how a Ruby class would
> map
> > to said C struct.
>
> Yes, I was simply looking for something like
>
> struct foo {
>  int bar;
>  char *baz;
> }
>
> cFoo = rb_define_class('cFoo')
> rb_map_struct(cFoo, foo)
> rb_accessor(cFoo, 'int', 'bar', bar)
> rb_accessor(cFoo, 'str', 'bar', bar)
>
> VALUE rb_function() {
>  foo* a = some_function_call()
>  return rb_deserialize_data(foo)
> }
>
> and from ruby
>
> foo = rb_function
> foo.bar
> foo.baz
>
> martin
>
>
You are attacking your problem from the wrong end. The problem is that C
doesn't have reflection so there is no way the rb_map_struct concept would
ever work. C won't tell you what the members are so you can't map it to
anything.

However, working in the other direction - you could use the C preprocessor
to build both your structs and the appropriate C code to define your ruby
class you want for each struct. It's not pretty but it does work - have a
look here for a nice defintion of what you are looking for
http://en.wikipedia.org/wiki/C_preprocessor#X-Macros

John

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