[Rcpp-devel] Working with strings

Le 17/11/12 15:22, Hadley Wickham a ?crit :

Hi all,

Am I doing something wrong with the functions below, or has this
behaviour not yet been implemented in Rcpp?

     cppFunction('std::string first_char(const CharacterVector x) {
       return x[0];
     }')

Ah.

    cppFunction('std::string first_char(CharacterVector x) {

+       return x[0] ;
+     }')
filee454ce15417.cpp: In function ?std::string
first_char(Rcpp::CharacterVector)?:
filee454ce15417.cpp:7: error: conversion from
?Rcpp::internal::string_proxy<16>? to non-scalar type ?std::string?
requested


Because of the way character vectors are implemented, we had to use a proxy
mechanism. so the result of the operator[] is a:

string_proxy<16>

        which has these two conversion operators:

                operator SEXP() const
                        this just retrieves the underlying CHARSXP using
STRING_ELT

                operator /*const */ char*() const
                        this also applies CHAR() to it to get a C string.



I think we tried having an automatic conversion to std::string, but this
failed and the compiler was confused.


Anyway. Short of automatic conversion, you can use a forced one:

    cppFunction('std::string first_char(const CharacterVector x) {
      return std::string( x[0] );
    }')

it works because of "only distance 1" conversion to go from string_proxy and
something (in that case char*) the ctor of string will accept.

     cppFunction('CharacterVector miss_c() {
       return CharacterVector::create(NA_STRING);
     }')

I've seen this too and was about to find my ways to a fix.

This works though:

cppFunction('CharacterVector miss_c() {
      CharacterVector out(1) ;
      out[0] = NA_STRING ;
      return out ;
    }')

Not as convenient. :-/