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[Rcpp-devel] Working with strings

5 messages · Hadley Wickham, Romain Francois

Original
Hadley Wickham
# 
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(CharacterVector x) {
      return x[0];
    }')

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

Hadley
Romain Francois
# 
Le 17/11/12 15:22, Hadley Wickham a ?crit :
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.
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. :-/
Hadley Wickham
# 
On Sat, Nov 17, 2012 at 9:23 AM, Romain Francois
<romain at r-enthusiasts.com> wrote:
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.
Ok, great - thanks!
Yup, that's what I've been using.

Hadley
Romain Francois
# 
Le 18/11/12 15:35, Hadley Wickham a ?crit :
Really not sure. We could easily add explicit conversions, e.g. as 
member functions:

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

or some other syntax, but automatic brings lots of compiler issues.

  
    

      
      
    

  


  


      

    

    

      
      

        


  

        

      Romain Francois
    

    

      
      #
    

  


  

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

            

      
      
      
    

        
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This should now work in rev 3990. Thanks for reporting the issue.