[ruby-talk:01875] object creation

Andrew Hunt writes:
> 
> Hi all,
> 
> Just a quick question -- is the following sequence the correct
> way to create a Ruby object from C?
> 
>   NEWOBJ(obj, c-type)  -- allocate memory for an object
>   OBJSETUP(obj, class, T_class); -- set appropriate flags
>   rb_obj_call_init(obj, argc, argv); -- call initialize

Depends on what you want to do...

AFAIK, this is the correct way to create an instance of a Ruby
built-in class. But many of them offer also a instance creation
function on C level, e.g. rb_float_new, rb_str_new2, rb_ary_new, ...

They will deliver a ready-to-use instance without additional taxes ;-)

But if you want to create an instance of an own class (e.g. a Ruby
extension written by yourself), you should going this way:

    my_RFoo *foo;
    VALUE obj;

    obj = Data_Make_Struct(klass, my_RFoo, mark_foo, free, foo);

    rb_obj_call_init(obj, argc, argv);

The function 'mark_foo' has to be written, if you store references to
Ruby class instances within your my_RFoo structure. This function
would be called during garbage collection by the GC to let you mark
all instances still alive yet! If your my_RFoo realizes a kind of
e.g. list, you would need such 'mark_foo' function. If you have no
reference to other Ruby instances stored within your my_RFoo
structure, you simply can pass a 0 (ZERO) instead.

The 'free' function would be called by the GC if it has decided that
the lifetime of your object has ended! Often a simple C 'free' is
enough! But if you wants to deallocate memory you have allocated
within the 'new' method, you have also provide an own deallocate
function, and pass it to Data_Make_Struct.

> 
> There was some discussion a while back on whether *every* class called 
> initialize or not, how did that end up?  It would apper from the
> current code that most of the built-in classes *do not* call
> an initialize method.

I think, we was not coming to a conclusion yet! *I* think, that
rb_obj_call_init should be called every time during instance
creation. But there may be some exceptions for classes like Fixnum,
for example.

This would make it simplier to derive a new class coded in Ruby from a
parent class coded in C, IMHO.

To simulate inheritance in C also, I moved the C structure creation
into my initialize method. The class will be created in MyFoo::new,
but the memory of the underlaying C data structure will be allocated
in MyFoo.initialize.

That has the advantage, that a child class of MyFoo, derived on C
level, can allocate its own memory, and let MyFoo.initialize
initialize it with the concerning data.

> 
> Many thanks!
> 
> /\ndy
> 

...

\cle

-- 
Clemens Hintze  mailto: [e-mail]