[Rcpp-devel] Passing template class as argument in Rcpp function

[Iñaki Ucar]

On Thu, 4 Mar 2021 at 17:39, Subhomoy Ghosh <subhomoy25 at gmail.com> wrote:
>
> Hi,
>
>   This is related to my post. My question is in the same spirit except for one additional complexity. How can one create a new instance of a template class object with a pointer on it and create an external pointer that can be further passed as an argument of a function? Here is a small example of my goal that will obviously not run and show "Use of class template Uniform2 requires template arguments".

You can create a non-template parent class (e.g., Distribution) from
which your template class derives. Then you just return a pointer to
the parent. In [1] you'll see an example of this (Activity is the
parent class; Seize, Release and the others are template classes that
inherit from Activity). In your example, you can use something like

Rcpp::XPtr<Distribution> ptr(new Uniform2<double>(as<double>(max), D ), true);

[1] https://github.com/r-simmer/simmer/blob/master/src/activity.cpp

Iñaki

> // template class
> template <typename T>
> class Uniform2 {
>
> public:
>   Uniform2(T max_,mat D_) :
>   max(max_), D(D_) {}
>
>   ~Uniform2(){};
>
>   T max;
>   mat D;
> };
>
>
> /// get function
> // [[Rcpp::export]]
> XPtr<Uniform2> getUniform(const mat &D, SEXP max,char type) {
>   // create pointer to a template Uniform object and
>   // wrap it as an external pointer
>   if(type=='double') {
>           Rcpp::XPtr<Uniform2> ptr(new Uniform2<double>(as<double>(max), D ), true);
>           return ptr;
>   } else if(type=='int') {
>           Rcpp::XPtr<Uniform2> ptr(new Uniform2<int>(as<int>(max), D ), true);
>           return ptr;
>    }
>   // return the external pointer to the R side
>   return Rcpp::XPtr<Uniform2>(R_NilValue);
> }
>
>
>
> //do something
> // [[Rcpp::export]]
> double test2(double z, XPtr<Uniform2> xp) {
>
>   double CC= z * xp->max;
>
>   return CC;
>
> }
>
> Thanks,
> Subhomoy
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-- 
Iñaki Úcar