[Rcpp-devel] Working with strings

[Hadley Wickham]

On Sat, Nov 17, 2012 at 9:23 AM, Romain Francois
<romain at r-enthusiasts.com> wrote:
> Le 17/11/12 15:22, Hadley Wickham a écrit :
>>
>> 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.

Do you think it's likely that you'll ever manage to get automatic
conversion to work?  That'll help me figure out the best way to write
about it.

>>      cppFunction('CharacterVector miss_c() {
>>        return CharacterVector::create(NA_STRING);
>>      }')
>
>
> I've seen this too and was about to find my ways to a fix.

Ok, great - thanks!

> This works though:
>
> cppFunction('CharacterVector miss_c() {
>       CharacterVector out(1) ;
>       out[0] = NA_STRING ;
>       return out ;
>     }')
>
> Not as convenient. :-/

Yup, that's what I've been using.

Hadley

-- 
RStudio / Rice University
http://had.co.nz/