int <FUNCTION_NAME> (realtype t, N_Vector y, N_Vector ydot, void *user_data)

time_t <- c(0.0, 0.4, seq(from = 10.4, len = 12, by = 10)) # time vector
cvode(time_t, c(1,0,0), my_fun, 1e-04, c(1e-08, 1e-08, 1e-08))

if (fstr == "<USER_DEFINED_FUNCTION_NAME>")
  XPtr<funcPtr> fun(new funcPtr(&<USER_DEFINED_FUNCTION_NAME>), true);
else
  Rcpp::Rout << "Supply the correct function name" << std::endl;

#include <Rcpp.h>
using namespace Rcpp;

#include <cvode/cvode.h>               /* prototypes for CVODE fcts., consts. */
#include <nvector/nvector_serial.h>    /* serial N_Vector types, fcts., macros */
#include <cvode/cvode_dense.h>         /* prototype for CVDense */
#include <sundials/sundials_dense.h>   /* definitions DlsMat DENSE_ELEM */
#include <sundials/sundials_types.h>   /* definition of type realtype */

int test (realtype t, N_Vector y, N_Vector ydot, void *user_data){

  // static keyword before int is not really required here

  NV_Ith_S(ydot,0) = -0.04 * NV_Ith_S(y,0) + 1e04 * NV_Ith_S(y,1) * NV_Ith_S(y,2);
  NV_Ith_S(ydot,2) = 3e07 * NV_Ith_S(y,1) * NV_Ith_S(y,1);
  NV_Ith_S(ydot,1) = -NV_Ith_S(ydot,0) - NV_Ith_S(ydot,2);

  return(0);

}

// declare funcPtr as a type for function pointer to a function with the
// signature same as function which describes the RHS for ODEs
// see reference here - http://gallery.rcpp.org/articles/passing-cpp-function-pointers/
typedef int (*funcPtr)(realtype t, N_Vector y, N_Vector ydot, void *user_data);

// [[Rcpp::export]]
XPtr<funcPtr> putFunPtrInXPtr() {

  // return(XPtr<funcPtr> (new funcPtr(&test)));
    XPtr<funcPtr> testptr(new funcPtr(&test), true);
    return testptr;

}