From: Charles Mills <cmills@...>
Date: 2005-09-29T01:51:44+09:00
Subject: Re: delayed initialization with alloc_function

Duncan Mac-Vicar Prett wrote:
> Hello guys, I am wrapping a C++ library and after changing from define
> singleton method "new" to alloc function and initialize, I ran into this
> problem, imagine the classes are like those:
>
> class MyOtherClass
> {
> public:
>         MyOtherClass(int a, int b);
> private:
>         int a;
>         int b;
> }
>
> class MyClass
> {
> public:
>         MyClass(int a, int b);
>         MyOtherClass myFunc(int c);
> private:
>         int a;
>         int b;
> };
>
> 1.- in alloc function, I have to create a MyClass object and wrap it with
> Data_Wrap_Struct, but I cant create an object at that time, because I have no
> constructor parameter until initialize (and I dont have a default constructor
> either). How do I solve this?
> 2.- If class MyClass func returns a MyOther class object, whats the best way
> to wrap this?, I have even a function that returns a collection of
> MyOtherClass objects, so I think I have to iterate trough it, and create a
> ruby wrapped object not from scratch but wrapping the current object in the
> collection, and then add all the ruby objects to a Ruby array, in this case,
> whats the best way to wrap it?
>
> Thanks for the help guys!
>
> Duncan

You have two options:
1) Don't use an alloc/initialize and instead override 'new'.  This is
simple, but it does not follow the ruby way, nor does it allow your
class to be subclassed.
2) Make a dummy alloc function, then do all the work in initialize.
(This is my preference.)
Your alloc function would look like:
VALUE
my_alloc(VALUE klass) {
  return Data_Wrap_Struct(klass, NULL, NULL, NULL);
}
and your initialize function like:
VALUE
my_init(int argc, VALUE *argv, VALUE self)
{
  MyOtherClass *p;
  /* handle args, create and initialize MyOtherClass instance */
  RDATA(self)->dmark = my_mark_function;
  RDATA(self)->dfree = my_free_function;
  RDATA(self)->data = p;
  return self;
}
You also should check RDATA(self)->data != NULL in all your methods.
This is easy though:
#define GetMyOtherClass(obj, var) do { \
  Data_Get_Struct(obj, MyOtherClass, var); \
  if ((var) == NULL) rb_raise(rb_eFatal, "you never initialized me"); \
} while (0)

You may also want to define a replace function (see array.c or
string.c).

--I am not much of a C++ programmer, so syntax may be wrong--

-Charlie