In Rcpp, how to get a user-defined structure from C into R

Question

Am using Rcpp packages and can get my C function to compile and run in R, but now I want to return a large, user-defined data structure to R. The fields in the structure are either numbers or strings - no new or odd types within the structure. The example below is simplified and doesn't compile, but it conveys the idea of my problem.

typedef struct {
  char*    firstname[128];
  char*    lastname[128];
  int      nbrOfSamples;
} HEADER_INFO;

// [[Rcpp::export]]
HEADER_INFO* read_header(Rcpp::StringVector strings) {
  FILE *fp;
  MEF_HEADER_INFO *header;
    
  char * filename = (char*)(strings(0));
  char * password = (char*)(strings(1));
    
  header = (HEADER_INFO*)malloc(sizeof(HEADER_INFO));
  memset(header, 0, sizeof(HEADER_INFO));
    
  fp = fopen(filename, "r");
  (void)read_header(header, password);
  return header;
}

I'm pretty sure that I could package the entries in the header back into a StringVector, but that seems like a brute-force approach. My question is whether a more elegant solution exists. It is not clear to me what form such a structure would even have in R: a named List?

Thanks!

Answer 1

The right structure in R depends on what your struct looks like exactly. A named list is the most general one. Here a simple sample implementation for a wrap function as referred to in the comments:

#include <RcppCommon.h>

typedef struct {
  char*   firstname[128];
  char*   lastname[128];
  int      nbrOfSamples;
} HEADER_INFO;

namespace Rcpp {
  template <>
  SEXP wrap(const HEADER_INFO& x);
}

#include <Rcpp.h>

namespace Rcpp {
  template <>
  SEXP wrap(const HEADER_INFO& x) {
    Rcpp::CharacterVector firstname(x.firstname, x.firstname + x.nbrOfSamples);
    Rcpp::CharacterVector lastname(x.lastname, x.lastname + x.nbrOfSamples);
    return Rcpp::wrap(Rcpp::List::create(Rcpp::Named("firstname") = firstname,
                      Rcpp::Named("lastname") = lastname,
                      Rcpp::Named("nbrOfSamples") = Rcpp::wrap(x.nbrOfSamples)));
  };
}

//  [[Rcpp::export]]
HEADER_INFO getHeaderInfo() {
  HEADER_INFO header;
  header.firstname[0] = (char*)"Albert";
  header.lastname[0] = (char*)"Einstein";
  header.firstname[1] = (char*)"Niels";
  header.lastname[1] = (char*)"Bohr";
  header.firstname[2] = (char*)"Werner";
  header.lastname[2] = (char*)"Heisenberg";
  header.nbrOfSamples = 3;
  return header;
}

/*** R
getHeaderInfo()
 */

Output:

> getHeaderInfo()
$firstname
[1] "Albert" "Niels"   "Werner"

$lastname
[1] "Einstein"   "Bohr"       "Heisenberg"

$nbrOfSamples
[1] 3

However, for this particular case a data.frame would be more natural to use, which can be achieved by replacing above wrap with:

  template <>
  SEXP wrap(const HEADER_INFO& x) {
    Rcpp::CharacterVector firstname(x.firstname, x.firstname + x.nbrOfSamples);
    Rcpp::CharacterVector lastname(x.lastname, x.lastname + x.nbrOfSamples);
    return Rcpp::wrap(Rcpp::DataFrame::create(Rcpp::Named("firstname") = firstname,
                                              Rcpp::Named("lastname") = lastname));
  };

Output:

> getHeaderInfo()
  firstname   lastname
1    Albert   Einstein
2     Niels       Bohr
3    Werner Heisenberg