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

[Rémi Lebret]

Hi Douglas,

I've tried that: Rcpp::S4 res(Rcpp::clone(Rcpp::S4(obj.slot("results")));

But it doesn't solve the problem because you actually clone the reference returned by the copy constructor...

Rémi

Le 16 févr. 2012 à 17:25, Douglas Bates a écrit :

> 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"));

Rémi Lebret
Ingénieur de recherche CNRS
Laboratoire de mathématiques de Lille1 - UMR 8524
Batiment M3 - Bureau 322
Tel: 03 20 43 67 82
remi.lebret at math.univ-lille1.fr

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