From: Joel VanderWerf <vjoel@...>
Date: 2002-09-25T04:21:07+09:00
Subject: Re: Object-Oriented struct Model in C

William Djaja Tjokroaminata wrote:
> Hi,
> 
> In the Ruby-C world, we can make an association between a Ruby class and a
> C struct through the use of Data_Wrap_Struct().  But in the Ruby world, a
> Ruby class can inherit from a parent, which in turn inherits from its
> parent, and so on.  Suppose each class in this inheritance chain has
> different data in its struct.  Is the following a good model for the
> corresponding C struct's?
> 
>     typedef struct
>     {
>         /* pointer to parent */
>         void *parentData;
>         void (*parentMarkFunc) (void*);
>         void (*parentFreeFunc) (void*);
> 
>         /* class-specific data */
>         ....
>     } sClass_Any;
> 
> (Is there any reference on how to do object-orientation in C?  Well, C++
> is excluded for now.)

My turn to sound like a broken record...

The CShadow module (in cgenerator on RAA) gives you this functionality. 
It opts for time rather than space efficiency, so there is a lot of code 
duplication. However, it is managed for you, so this is only a problem 
if you want small executables.

Here's a simple example with inheritance:

   require 'cgen/cshadow'

   class Parent
     include CShadow

     shadow_attr_accessor :ruby_str => String
     shadow_attr_accessor :c_int => "int c_int"
   end

   class Child < Parent
     shadow_attr_accessor :obj => Object
   end

   Parent.commit
     # we're done adding attrs and methods, so make.

   x = Child.new
   x.ruby_str = "foo"
   x.obj = [1,2,3]
   x.c_int = 3

The generated code looks, in part, as follows. Note that 
CGenerator/CShadow produces quite readable C code; I have snipped but 
not changed anything. In the Parent.h header file we have:

   typedef struct Parent_Shadow {
       VALUE self;
       VALUE ruby_str; // String;
       int c_int;
   } Parent_Shadow;

   typedef struct Child_Shadow {

       /* Parent_Shadow members */
       VALUE self;
       VALUE ruby_str; // String;
       int c_int;

       VALUE obj; // Object;
   } Child_Shadow;

Again, members are duplicated, which would be awful if you wanted to 
manually maintain the code, but who wants to do that :-)

In the Parent.c file, we have:

   void mark_Parent_Shadow(Parent_Shadow *shadow)
   {
       rb_gc_mark((void *)shadow->self);
       rb_gc_mark((void *)shadow->ruby_str);
   }

   void mark_Child_Shadow(Child_Shadow *shadow)
   {
       rb_gc_mark((void *)shadow->self);
       rb_gc_mark((void *)shadow->ruby_str);
       rb_gc_mark((void *)shadow->obj);
   }

and similarly for the free functions, persistence functions, etc. The 
#new method generated by CShadow is where rb_obj_call_init() gets 
called. Note that initialization of shadow attributes is customizable.

   VALUE new_module_Child_singleton_method(int argc, VALUE *argv, VALUE 
self)
   {
       VALUE object;
       Child_Shadow *shadow;

       object = Data_Make_Struct(self,
                  Child_Shadow,
                  mark_Child_Shadow,
                  free_Child_Shadow,
                  shadow);
       shadow->self = object;

       shadow->ruby_str = Qnil;
       shadow->obj = Qnil;

       rb_obj_call_init(object, argc, argv);

       return object;
   }

The #ruby_str= method only needs to be defined for Parent, because, of 
course, Ruby itself handles inheritance of methods. Note the type 
checking, which is done for the ruby_str member but not for the obj member.

   VALUE ruby__str_equals_module_Parent_method(int argc, VALUE *argv, 
VALUE self)
   {
       VALUE arg;
       Parent_Shadow *shadow;

       rb_scan_args(argc, argv, "1", &arg);

       if (!NIL_P(arg) &&
           rb_obj_is_kind_of(arg, module_String) != Qtrue)
         rb_raise(module_TypeError,
                  "argument arg declared String but passed %s.",
                  STR2CSTR(rb_funcall(
                    rb_funcall(arg, ID_type, 0),
                    ID_to__s, 0)));

       Data_Get_Struct(self, Parent_Shadow, shadow);
       shadow->ruby_str = arg;
       return arg;
   }