How to wrap a C struct in a ruby extension

device_driver.cpp

#include "ruby.h"
// http://www.angelfire.com/electronic2/issac/rb_cpp_ext_tut.txt
typedef VALUE (ruby_method)(...);

VALUE DeviceDriver = Qnil;

struct conn {
  VALUE settings;
  connection *conn_ptr;
};

// Release memory used by struct
extern "C" void device_driver_free(conn *connection) {
  free(connection);
}

// Allocate needed memory for struct
extern "C" VALUE device_driver_alloc(VALUE self) {
  struct conn *connection = (conn*)malloc(sizeof(struct conn));

  return Data_Wrap_Struct(self, NULL, device_driver_free, connection);
}

// A basic initializer
extern "C" VALUE method_initialize(VALUE self, VALUE settings) {
  conn *connection;
  Data_Get_Struct(self, conn, connection);
  connection->settings = settings;

  return self;
}

// Метод который объявляет весь модуль
extern "C" void Init_device_driver() {
  DeviceDriver = rb_define_class("DeviceDriver", rb_cObject);

  rb_define_alloc_func(DeviceDriver, alloc_device_driver);
  // When transcompiling cpp to c code ruby_method macro has to be used 
  // for correct compiling
  rb_define_method(DeviceDriver, "initialize", (ruby_method*) &method_initialize, 1);
}