[Rcpp-devel] Idiom for accessing scalars

[Douglas Bates]

2012/1/7 Romain François <romain at r-enthusiasts.com>:
> 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.

Which, to me, is the disadvantage.  The asInteger function is brief,
understandable, flexible and well-tested.  This may look transparent
to you but not to many others.