How to use struct vector as an input parameter in Rcpp

Question

I need to supply a vector of structs to an Rcpp function. The cpp file is as below

#include <Rcpp.h>


using namespace Rcpp;

struct teststc{
  int y;
};


// [[Rcpp::export]]
void teststruct(std::vector<teststc>  ys){
  Rcpp::Rcout <<"test"<<std::endl;
}

When I compile this cpp file, it returns this error

error: no matching constructor for initialization of 'teststc'

I actually see some functions using similar input parameters. So, I don't know why mine doesn't work. Any help is greatly appreciated!

Answer 1

The problem is that you are trying to export a function to R that takes as an argument an object of type std::vector<teststc>. Such an object does not exist in R, so it cannot pass it to the C++ code.

If you want to be able to pass an object to C++ from R and convert it into your own struct, then you must write the C++ code that will convert an R object to your struct. Rcpp makes this much easier, but it is not magical, and cannot automatically convert an R object into an arbitrary struct without being told how you would like this to happen. You would therefore need to write the C++ code to do this.

For example, the following code will take an integer vector from R, convert it into a std::vector<teststc>, then print out the elements of that vector:

#include <Rcpp.h>
#include <vector>

using namespace Rcpp;

struct teststc {
  int y;
  teststc(int x) { this->y  = x;}
};

std::vector<teststc> make_testvec(IntegerVector iv) {
  std::vector<teststc> out;
  for(int i = 0; i < iv.size(); ++i) {
    out.push_back(teststc((int) iv[i]));
  }
  return(out);
}

// [[Rcpp::export]]
void teststruct(IntegerVector ys) {
  std::vector<teststc> stc = make_testvec(ys);
  for(unsigned i = 0; i < stc.size(); ++i) {
   Rcout << stc[i].y << ' '; 
  }
  Rcpp::Rcout << std::endl;
}

So that back in R I can do:

teststruct(1:5)
#> 1 2 3 4 5