From: Todd Fisher <todd.fisher@...>
Date: 2008-07-24T11:24:46+09:00
Subject: Re: calling c++ from Ruby

Mr_Tibs wrote:
> Hi,
> 
> I wrote this question on the newsgroup yesterday, but I didn't see it
> actually get posted. Here it goes again. I'm trying to call a c++
> function from Ruby and I'm getting some errors compiling the c++
> part.
> 
> Here is my code:
> #include "ruby.h"
> #include <iostream>
> #include <stdarg.h>
> VALUE MyTest = Qnil;
> extern "C" VALUE create_project(char *c, ...)
> {
>         va_list args;
>         va_start(args, c);
> }
> extern "C" void Init_mytest()
> {
>         MyTest = rb_define_module("MyTest");
>         rb_define_method(MyTest, "create_project", &create_project,
> 1);
> }
> 
> And here is the error:
> MyTest.cpp: In function �void Init_mytest()�:
> MyTest.cpp:16: error: invalid conversion from �VALUE (*)(char*, ...)�
> to �VALUE (*)(...)�
> MyTest.cpp:16: error:   initializing argument 3 of �void
> rb_define_method(VALUE, const char*, VALUE (*)(...), int)�
> 
> If I don't put "char *c" in the arguments of create_project, it
> compiles and works just fine. But if I don't have it, how can I get
> the arguments?
> I realize this is more of a c++ question, but if anyone has any
> simple, fully working example of calling c++ code from Ruby, I would
> like to see it.
> 
> Thanks,
> Tiberiu

Hi,

I think you meant to use rb_define_singleton_method, instead of 
rb_define_method.

Also, the method signature of your C/C++ function is important.  If you 
want to use variable number of arguments, it needs to look something 
like this:

VALUE your_method_name(int argc, VALUE *argv, VALUE klass)

Also, when using C++ you need to cast to correct types... so, in your 
case you could fix the compile time error by casting:

 rb_define_method(MyTest, "create_project", (VALUE 
(*)(...))&create_project, -1);

Also, when using variable arguments you need to pass a -1, since it's an 
unknown number of arguments.  Ruby also has it's own set of methods to 
use to access the arguments.

Have a look at rb_scan_args, 
http://www.oreillynet.com/ruby/blog/2007/04/c_extension_authors_use_rb_sca_1.html

Hope this helps,
-Todd
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