convert Rcpp::CharacterVector to std::string

Question

I am trying to open a file within an Rcpp function, so I need the file name as a char* or std::string.

So far, I have tried the following:

#include <Rcpp.h> #include <boost/algorithm/string.hpp> #include <fstream> #include <string>  RcppExport SEXP readData(SEXP f1) {     Rcpp::CharacterVector ff(f1);     std::string fname = Rcpp::as(ff);     std::ifstream fi;     fi.open(fname.c_str(),std::ios::in);     std::string line;     fi >> line;     Rcpp::CharacterVector rline = Rcpp::wrap(line);     return rline; } 

But apparently, as does not work for Rcpp::CharacterVector as I get a compile time error.

foo.cpp: In function 'SEXPREC* readData(SEXPREC*)': foo.cpp:8: error: no matching function for call to 'as(Rcpp::CharacterVector&)' make: *** [foo.o] Error 1 

Is there a simple way to get a string from the argument or somehow open a file from the Rcpp function argument?

Answer 1

Rcpp::as() expects a SEXP as input, not a Rcpp::CharacterVector. Try passing the f1 parameter directly to Rcpp::as(), eg:

std::string fname = Rcpp::as(f1);  

Or:

std::string fname = Rcpp::as<std::string>(f1);  

Answer 2

The real issue is that Rcpp::as requires that you specify the type you want to convert to manually, such as Rcpp::as<std::string>.

The input of all as overloads is always a SEXP so the compiler does not know which one to use and cannot make the decision automatically. that's why you need to help it. Things work differently for wrap which can use the input type to decide which overload it will use.