[Rcpp-devel] gc() calls destructor twice when both class and XPtr deleted

[Dirk Eddelbuettel]

On 11 February 2013 at 12:16, Michael Shvartsman wrote:
| Hi Rcpp-devel -
| 
|      In some of my code, I return an XPtr pointing to one C++ class 
| (e.g. class A) to the R side, and then pass it back down to the C++ side 
| in the constructor of another class (e.g. class B, which needs access to 
| some methods in class A). If I rm() both class A and the pointer to it 
| on the R side (or they otherwise both go out of scope), then the garbage 
| collector calls the destructor of class A twice and attempts to free the 
| memory twice.
| 
|      Happens on OSX and Ubuntu with latest R and Rcpp. Is this intended 
| behavior? If yes, what's a recommended workaround? I can go into more 
| detail on why this is useful to my workflow if needed. Below is a 
| minimal example. Best,

Thanks for sending a complete example -- very helpful.

I will think about this some more, but my first gut reaction is that your
design is wrong.  You are essentially "just" using XPtr to get two references
to the same memory, and then for all intends and purposes doing

    x = new Something;
    delete x;
    delete x;

which also doesn't work.  I have worked quite happily with XPtr in the past,
and so have others.  I have however not mixed them with Modules, I think, so
maybe that enters.

But I think the best answers to the "I hurts when I do this" remains "well
then just don't it".  

And I may well have missed something here...

Dirk

| 
|          Mike Shvartsman.
| 
| -----
| library(Rcpp)
| library(inline)
| 
| inc <- '
| using namespace Rcpp;
| class A{
|      public:
|          A();
|          ~A();
|          XPtr<A> getPtr();
| };
| 
| XPtr<A> A::getPtr(){
|      return(XPtr<A>(this));
| }
| 
| A::A(){
|      Rcout << "CTOR" << std::endl;
| }
| 
| A::~A(){
|      Rcout << "DTOR" << std::endl;
| }
| 
| RCPP_MODULE(mod){
|      class_<A>("A")
|      .constructor()
|      .method("getPtr", &A::getPtr)
|      ;
| }
| '
| 
| func <- cxxfunction(signature(), include=inc, plugin='Rcpp')
| 
| mod <- Module('mod', getDynLib(func))
| 
| 
| A <- mod$A
| 
| a <- new(A)
| aPtr <- a$getPtr()
| rm(a)
| rm(aPtr)
| gc()
| -----
| 
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-- 
Dirk Eddelbuettel | edd at debian.org | http://dirk.eddelbuettel.com