[Rcpp-devel] Idiom for accessing scalars

[Romain François]

Le 06/01/12 20:46, Douglas Bates a écrit :
> On Fri, Jan 6, 2012 at 1:12 PM, Dirk Eddelbuettel<edd at debian.org>  wrote:
>> On 6 January 2012 at 12:59, Douglas Bates wrote:
>> | On Fri, Jan 6, 2012 at 12:39 PM, John Chambers<jmc at stat.stanford.edu>  wrote:
>> |>  The "Rf_" part of the API in particular is ugly and somewhat of an add-on
>> |>  forced in a few examples by the use of some common names in the macro files.
>> |
>> | But, as it stands, that is a requirement when using Rcpp.
>>
>> Where?  I can think of one propagated use, which is at the bottom of the
>> try/catch structure where we use ::Rf_error.  But the commonly used macros
>> hide it, and we could/should obviously wrap this.
>>
>> Otherwise, and especially since the 'Rcpp sugar' initiative took off, I don't
>> really touch any ::Rf_* myself anymore.  Inside the Rcpp code base, sure. But
>> not really in user-facing stuff and Rcpp applications.
> I didn't make myself clear.  What I meant was that it is not possible
> to use asInteger in Rcpp and count on the name being remapped to
> Rf_asInteger.
>
>> | I think of the Rf_ part as more due to the fact that C doesn't have a
>> | concept of namespaces so anything in the R API is at the top level
>> | namespace leading to some conflicts.
>>
>> Agreed.  But speaking stylistically, for the same reason that we prefer C++
>> versions of C header files (eg cstdint over stdint.h, cstdio over stdio.h,
>> ...) I am with John on the preference for C++ idioms when given a choice.
> I suppose I could have just checked whether Rcpp::as<int>  calls
> Rf_asInteger.  If so, everything is cool.  Unfortunately, I haven't
> been able to find that specialization.
>
as lives in the inst/include/Rcpp/as.h file, and we have to follow 
template wizardry:

it starts from :

template <typename T> T as( SEXP m_sexp) {
         return internal::as<T>( m_sexp, typename 
traits::r_type_traits<T>::r_category() ) ;
     }

with T=int, so we end up calling this one:

template <typename T> T as( SEXP x, ::Rcpp::traits::r_type_primitive_tag ) {
             if( ::Rf_length(x) != 1 ) throw ::Rcpp::not_compatible( 
"expecting a single value" ) ;
             const int RTYPE = ::Rcpp::traits::r_sexptype_traits<T>::rtype ;
             SEXP y = PROTECT( r_cast<RTYPE>(x) );
             typedef typename ::Rcpp::traits::storage_type<RTYPE>::type 
STORAGE;
             T res = caster<STORAGE,T>( *r_vector_start<RTYPE,STORAGE>( 
y ) ) ;
             UNPROTECT(1) ;
             return res ;
         }


which does the magic. There is no calls to asInteger.

Romain

-- 
Romain Francois
Professional R Enthusiast
http://romainfrancois.blog.free.fr