[Rd] Linking to native routines in other packages

[Romain Francois]

Hello,

I'm currently working on making Rcpp use the feature described here more:
http://cran.r-project.org/doc/manuals/R-exts.html#Linking-to-native-routines-in-other-packages

To give more context, Rcpp has for a long time built what we called "the 
Rcpp user library", i.e. a library we could link against user the 
linker. We were then producing appropriate linker flag with 
Rcpp:::LdFlags(), ...

Now, I'm moving away from this and the intention is that a package using 
Rcpp would only have to use

LinkingTo: Rcpp

This sets the -I flag as before to find headers from Rcpp, but this also 
now takes advantage of what is described in writing R extensions:
http://cran.r-project.org/doc/manuals/R-exts.html#Linking-to-native-routines-in-other-packages

I'm doing this in a way that, when we are not compiling Rcpp, for 
example the "type2name" function is defined in Rcpp's headers as an 
inline function that grabs the registered function from Rcpp.

inline const char* type2name(SEXP x){
         typedef const char* (*Fun)(SEXP) ;
         static Fun fun = GET_(Fun) R_GetCCallable( "Rcpp", "type2name") ;
         return fun(x) ;
     }


This works great.


Now for the question. The documentation says:

"It must also specify ‘Imports’ or ‘Depends’ of those packages, for they 
have to be loaded prior to this one (so the path to their compiled code 
has been registered)."

Indeed if I don't have Depends or Imports, the R_init_Rcpp is not 
called, and so the function is not registered.

But now if I do Depends: Rcpp or Imports: Rcpp for the sole purpose of 
this LinkingTo mechanism, I'm getting

* checking dependencies in R code ... NOTE
Namespace in Imports field not imported from: ‘Rcpp’
   All declared Imports should be used.
See the information on DESCRIPTION files in the chapter ‘Creating R
packages’ of the ‘Writing R Extensions’ manual.

It would be simple enough to require of our users that they have 
Imports: Rcpp and import(Rcpp) or less in their NAMESPACE, but I was 
wondering if we could make this more transparent, i.e. having LinkingTo: 
Rcpp mean loading Rcpp.

I'm also curious about this sentence from the doc:

"In the future R may provide some automated tools to simplify exporting 
larger numbers of routines."

Is there a draft of something ?

Romain



For details on how we will be using LinkingTo. Please see:

https://github.com/RcppCore/Rcpp/blob/master/inst/include/Rcpp/routines.h

where depending
- when we are compiling Rcpp, we just have a declaration of the 
function, which is then defined in any of our .cpp files.
- when we are using Rcpp from another package, we are retrieving the 
function

https://github.com/RcppCore/Rcpp/blob/master/src/Rcpp_init.cpp

where the functions are registered with the RCPP_REGISTER macro.

This way of using it moves all the logic to the package exposing its 
functions. I find this nicer to use than other tactics I've seen, such 
as the sub technique from Matrix ...

-- 
Romain Francois
Professional R Enthusiast
+33(0) 6 28 91 30 30