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

On Thu, Feb 16, 2012 at 9:25 AM, R?mi 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"));