[Rcpp-devel] Module with vector of a class with inheritance, how to avoid slicing

Wed Apr 3 20:07:14 CEST 2013

additional remark : if operator SEXP() ; is added to class father,  children1_t,  and children2_t the solution with XPtr compiles but gives the same result as the solution with shared_ptr (i.e. slicing)
R.

----- Mail original -----
De: "Robin Girard" <robin.girard at mines-paristech.fr>
À: rcpp-devel at lists.r-forge.r-project.org
Cc: "Robin GIRARD" <robin.girard at mines-paristech.fr>
Envoyé: Mercredi 3 Avril 2013 18:43:32
Objet: Module with vector of a class with inheritance, how to avoid slicing

I am still working on my vector of a class father with inheritance (polymorphic subclasses ?). I created an exemple below that works fine but I face the known problem of "slicing" as named here : http://stackoverflow.com/questions/10154977/c-vector-with-inheritance and fail to implement the proposed solution.

I have 2 tried 2 things and ended up with 2 questions: 

Question 1 -  I tryed the solution proposed in the link with make_shared and shared_ptr. I had to add PKG_CXXFLAGS=-g -std=c++0x since my MinGW compiler did not find any -std=c++11.  I got no compilation error but when I run the code it slices my children class : 

> res=new("Rcpp_vector_Of_father")
> res$push_back(new("Rcpp_father"))
> res$push_back_children1_t(new("Rcpp_children1_t"))
> res$WhoAmI(1)
father
> res$WhoAmI(0)
father
> 
> res[[1]]$WhoAmI()
father
> res[[0]]$WhoAmI()
father

Question 2 - I tryed a solution with XPtr (although I'm not sure how this works) and got the following error at the module compilation 
example_mod_consumption.cpp: In member function 'void vector_Of_father::push_back_children1_t(children1_t)':
example_mod_consumption.cpp:80:53: error: no matching function for call to 'std::vector<Rcpp::XPtr<father> >::push_back(Rcpp::XPtr<children1_t>)'

below are the code for the two approach and at the end the code of the module. 

-------------------- Solution with XPtr 

#include <Rcpp.h>

using namespace std;
using namespace Rcpp;

class father ;
class children1_t;
class children2_t ;
class vector_Of_father;

RCPP_EXPOSED_CLASS(father)
RCPP_EXPOSED_CLASS(children1_t)
RCPP_EXPOSED_CLASS(children2_t)
RCPP_EXPOSED_CLASS(vector_Of_father)

class father
{
  public:
    ~father(){};
    father(){};
    father(father const & x) {};
    void WhoAmI() const{
      Rcout<<"father"<<endl;
    };
};

class children1_t : public father
{

  public:
    ~children1_t(){};
    children1_t(){};
    children1_t(children1_t const & x){};
    void WhoAmI() const{
      Rcout<<"son1"<<endl;
    };
};

class children2_t : public father
{
  public:
    ~children2_t(){};
    children2_t(){};
    children2_t(children2_t const & x){};
        void WhoAmI() const{
      Rcout<<"son2"<<endl;
    };
};

class vector_Of_father {

  public:
    std::vector<XPtr<father> > MyfatherList_;
    //std::vector<shared_ptr<father> > MyfatherList_;
    ~vector_Of_father(){};
    vector_Of_father() : MyfatherList_(){};
	father vec_get( int i) { return(*(MyfatherList_.at(i))); };

	void WhoAmI(int i) const
	{
		MyfatherList_[i]->WhoAmI(); 
	};
	int size(){ return(MyfatherList_.size()); };

	void push_back(father func){
		MyfatherList_.push_back(XPtr<father>(&func));
	};

	void push_back_children1_t(children1_t func){
		MyfatherList_.push_back(XPtr<children1_t>(&func));
	};
};

-------------------- Solution with make_shared and shared_ptr

only the class vector_of_father is different also #include <memory> is added after Rcpp.h include. 

class vector_Of_father {

  public:
    std::vector<shared_ptr<father> > MyfatherList_;
    ~vector_Of_father(){};
    vector_Of_father() : MyfatherList_(){};

  father vec_get( int i) { return(*(MyfatherList_.at(i))); };

  void WhoAmI(int i) const
  {
    MyfatherList_[i]->WhoAmI(); 
  };
  int size(){ return(MyfatherList_.size()); };

  void push_back(father func){
    MyfatherList_.push_back(make_shared<father>(func));
  };
  void push_back_children1_t(children1_t func){
    MyfatherList_.push_back(make_shared<children1_t>(func));
  };
};

-------------- the module code

RCPP_MODULE(mod_example2){
  using namespace Rcpp;

  class_<father>( "father" )
	//constructors  
	  .constructor()
    .method("WhoAmI",&father::WhoAmI)
	;

  class_<children1_t>( "children1_t" )
    .derives<father>("father" )
	//constructors
	  .constructor()
    .method("WhoAmI",&children1_t::WhoAmI)
	;

  class_<children2_t>( "children2_t" )
    .derives<father>("father" )
	//constructors
	  .constructor()
    .method("WhoAmI",&children2_t::WhoAmI)
	;

  class_<vector_Of_father>( "vector_Of_father")
    .constructor()
   // .constructor<int>()
    .method( "size", &vector_Of_father::size)
 // .method("capacity", &cplfunctionvec::capacity,"Return size of allocated storage capacity. Returns the size of the storage space currently allocated for the vector, expressed in terms of elements.")
//  .method( "max_size", &cplfunctionvec::max_size)
    .method( "WhoAmI",&vector_Of_father::WhoAmI )
    //.method( "test",&vector_Of_father::test )
    .method( "push_back", &vector_Of_father::push_back )
    .method("push_back_children1_t",&vector_Of_father::push_back_children1_t)
//  .const_method( "at", &cplfunctionvec::at )
    .method("[[",&vector_Of_father::vec_get)
  //  .method("[[<-",&vector_Of_father::vec_set)
  ;

}

Dr. Girard Robin
Chargé de Recherche

MINES-ParisTech / Département Energétique et Procédés / PERSEE / Groupe ERSEI
Centre Procédés, Energies Renouvelables et Systèmes Energétiques (PERSEE)
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