From: Tony Smith <tony@...>
Date: 2002-01-11T19:54:56+09:00
Subject: Re: Extension with C++

On Friday 11 January 2002 05:03, Mahina wrote:
> I'm a beginner in programming Ruby. 
> I want to use it for my works. 
> But i have a problem. I didn't understand, how extend Ruby with C++.
> Though i already read  chapter "Extending Ruby" in Programming Ruby.
> What can you advice me?

Marrying Ruby and C++ can be a little awkward as Dave and Andy mention in the 
book. The trick is to write most of your glue code in C++, but with C 
bindings i.e. not object oriented. That way, Ruby calls a regular "C" 
function, which can then invoke C++ methods on C++ objects ( or indeed Ruby 
methods on Ruby objects )

Check out:
   http://public.perforce.com:8080/guest/tony_smith/perforce/API/Ruby/main/

for my implementation of a Ruby extension to a C++ api. I'm sure there are 
other (and probably better) examples at the RAA too. 

A short snippet which illustrates the point. Taken from p4.cc:

extern "C"
{

static VALUE p4_new( VALUE pClass )
{
    VALUE       argv[ 1 ];
    P4ClientApi *p4 = new P4ClientApi();
    VALUE       self;

    self = Data_Wrap_Struct( pClass, p4_mark, p4_free, p4 );
    rb_obj_call_init( self, 0, argv );
    return self;
}

static VALUE p4_set_client( VALUE self, VALUE client )
{
    P4ClientApi *p4;
    Data_Get_Struct( self, P4ClientApi, p4 );
    p4->SetClient( STR2CSTR( client ) );
    return Qtrue;
}

void    Init_P4api()
{
    // Ruby instantiation
    eP4 = rb_define_class( "P4Exception", rb_eRuntimeError );

    cP4 = rb_define_class( "P4", rb_cObject );
    rb_define_singleton_method( cP4, "new", (VALUE (*)() ) p4_new, 0 );
    rb_define_method( cP4, "initialize", (VALUE (*)() )p4_init, 0 );
    rb_define_method( cP4, "client", (VALUE (*)() )p4_set_client, 1 );
    
    // Lots more method definitions...
}

} // Extern C 

You can see that the C++ object is created in p4_new() and wrapped in a Ruby 
object of the given class. The "C" functions (actually C++ but with unmangled 
names) then use Data_Get_Struct() to extract the C++ object from the Ruby 
object and call the appropriate method.

Hope that helps!

Tony