From saveez at hotmail.com  Mon Apr 18 11:20:16 2005
From: saveez at hotmail.com (Ali -)
Date: Mon Apr 18 11:20:25 2005
Subject: [Rd] RE: A 'true' R-wrapper for C++ classes
Message-ID: <BAY17-F344E23FE86622C7C9FD4DBD1290@phx.gbl>

Sorry about the mistake in the previous post, here is the corrected version:

Hello

I am trying to wrap some C++ classes into R.

(1) Comparing the OOP and methods packages, I have came to this conclusion
that OOP works much better for this wrapper -- please correct me if I am
wrong. One question is why this useful package (OOP) is not included in the
official release of R?

(2) Choosing the OOP package way, I have carried out the following steps to
wrap the C++ classes:

  (2.1) A C-wrapper is created to convert the C++ class to some C-style
code.
  (2.2) An R-wrapper wraps the C-wrapper.

Here is a rough example to demonstrate the above:

---------------------
C++ class:

class foo
{
public:
  foo();
  ~foo();

  fun();
}

---------------------
C-wrapper:

extern "C" SEXP R_foo_init()
{
  foo* ptr= new foo();

  SEXP res;
  PROTECT(res = R_MakeExternalPtr(ptr, R_NilValue, R_NilValue));
  UNPROTECT(1);

  return res;
}

extern "C" SEXP R_foo_fun(SEXP obj)
{
  foo *ptr= (foo *) R_ExternalPtrAddr(obj);
  ptr->fun();

  return  R_NilValue;
}

---------------------
R-wrapper:

defineClass(className = "foo");

vtkObject$defineFields(ptr = "externalptr");

vtkObject$defineMethod(
	"initialize",
	function(){
		ptr <- .Call("R_foo_init")
	}
);

vtkObject$defineMethod(
    "fun",
    function()
    {
        .Call("R_foo_fun", ptr);
    }
);
--------------------

(3) The above model lacks something like an 'environment' for the pointer to
the C++ object to live in it. Assume we create the foo class in R like:

  obj <- foo$new()

Now, the following would return an error:

  obj$fun()

and the reason is that the pointer created in the initialize method is lost.

(4) The question is how to assign an environment to the pointers. A well
described answer, rather than some abstract hints, is well-appreciated. Also
I am curious to know why there is no standard method for R to wrap C++
classes, something like JNI.

Thanks,

Ali


[Rd] RE: A 'true' R-wrapper for C++ classes
Ali -