[Rcpp-devel] Problem with the copy constructor of Rcpp::S4

[Douglas Bates]

On Thu, Feb 16, 2012 at 9:25 AM, Rémi Lebret
<remi.lebret at math.univ-lille1.fr> wrote:
> Hi,
>
> I've got some issues by using the copy constructor of the S4 class.
>
> I want to make a copy of a S4 object but it looks like the copy constructor
> returns a reference instead of a new instance of the S4 class.
>
> I made a dummy example to illustrate my problem:
>
> ##############################################################
> setClass(
>   Class="ResultObject",
>   representation=representation(
>     data = "numeric"
>   ),
>   prototype=prototype(
>     data = numeric(0)
>   )
> )
> ##############################################################
>
> ##############################################################
> setClass(
>   Class="TestObject",
>   representation=representation(
>     size = "numeric",
>     results = "ResultObject"
>   ),
>   prototype=prototype(
>     size = numeric(0),
>     results = new("ResultObject")
>   )
> )
> ##############################################################
>
>
> ##############################################################
> src <- '
>   Rcpp::S4 obj(S4Object);
>   // get the size
>   int n = obj.slot("size");
>   // get a copy of the ResultObject by calling the copy constructor
>   // --> but here it looks like a reference not a copy ?
>   Rcpp::S4 res(obj.slot("results"));
>
>   // if size is greater than create a list of results
>   if ( n > 1 ){
>     // create a new list
>     Rcpp::List resList(n);
>     // loop over the results
>     for (int i=0; i<n; i++ ){
>       // save results into the S4 object
>       res.slot("data") = i;
>       // copy the S4 object into the list -->  so it looks like a copy of a
> reference here...
>       resList[i]=res;
>     }
>     // set the list to our S4 object
>     obj.slot("results") = resList;
>   }
>   // otherwise store only one result
>   else if ( n == 1 ){
>     res.slot("data") = 1;
>      obj.slot("results") = res;
>   }
>   // return the S4 object
>   return obj;
> '
>
> testList <- cxxfunction(signature(S4Object="TestObject"), src, plugin =
> "Rcpp")
> ##############################################################
>
> # test with 3 elements
> # all data slot will contain the same value, that's not what we expect
> test1<-new("TestObject", size=3)
> testList(test1)
>
> # test with only one element
> test2<-new("TestObject", size=1)
> testList(test2)
>
>
>
> Any idea how to make a copy of a S4 object?

You probably want to clone the object, not use the copy constructor.
That is, (not tested)

Rcpp::S4 res(Rcpp::clone(obj.slot("results"));