[Rcpp-devel] Re-using C++ code from a package with modules and defining linker flags using Rscript

[Dirk Eddelbuettel]

Hi Jelmer,

On 15 May 2011 at 21:42, Jelmer Ypma wrote:
| Dear all,
| 
| This is a question about recommended ways of linking C++ code in an R
| package to C++ code defined in other R packages (using Rcpp 0.9.4.1

Well, you seem to make the assumption that this would be supported and
documented. But it is not really. 

| and Rtools on Windows). I built package A, which contains C++ classes
| and uses modules to expose some of the methods in these classes to R.
| Then I built package B, which uses some of the C++ classes in A
| directly. I added package A to the LinkingTo: and Depends: fields in
| DESCRIPTION. In order for R to find the header-files (.h) of package
| A, when compiling package B, I put those header files in
| A/inst/include. R-Functions LdFlags() and CxxFlags() are defined in
| package A, so I can have something like
| 
| PKG_LIBS = $(shell "${R_HOME}/bin${R_ARCH_BIN}/Rscript.exe" -e
| "Rcpp:::LdFlags()") \
|            $(shell "${R_HOME}/bin${R_ARCH_BIN}/Rscript.exe" -e "A:::LdFlags()")
| 
| in B/src/Makevars.
| 
| At this point I ran into problems when installing package B (error:
| failed to load module modA from package A). It turns out the same
| problem occurs when using a Rcpp skeleton package with modules.
| 
| library('Rcpp')
| Rcpp.package.skeleton( 'testmodA', module=TRUE )
| 
| with one file (e.g. inline.R) added in testmodA/R containing
| ===
| LdFlags <- function(print=TRUE) {
|     if (print) cat("test\n") else return( "test" )
| }
| ===
| 
| Install the package
| R CMD INSTALL testmodA
| 
| Including
| PKG_LIBS = $(shell "${R_HOME}/bin${R_ARCH_BIN}/Rscript.exe" -e
| "Rcpp:::LdFlags()") \
|            $(shell "${R_HOME}/bin${R_ARCH_BIN}/Rscript.exe" -e
| "testmodA:::LdFlags()")
| 
| in package B's Makevars leads to errors during compilation, which I
| traced back to:
| > Rscript -e "testmodA:::LdFlags()"
| 
| Error: .onLoad failed in loadNamespace() for 'testmodA', details:
|   call: value[[3L]](cond)
|   error: failed to load module yada from package testmodA
| Execution halted
| 
| If I replace this by
| Rscript -e "library(testmodA); testmodA:::LdFlags()"
| Loading required package: methods
| Loading required package: Rcpp
| test
| 
| it works as expected, so this is what I use in the Makevars file of package B.
| 
| Without modules in testmodA
| > Rscript -e "testmodA:::LdFlags()"
| works fine.
| 
| Is this the recommended way of adding linker and cxx flags to packages
| depending on other R packages with modules? I tried to look at

We simply show no example of definiting object code in one package and having
other packages use it -- with the exception of the Rcpp package itself.  And
there it takes quite some effort to make this happen.

You could look at what lme4 and Matrix do; this is documented in the 'Writing
R Extensions' manual under '[Section] 5.4 Registering native routines'.

| packages using RcppArmadillo or RcppGSL for inspiration, but both of
| these packages don't implement modules. Any other packages I should
| look at?

RcppArmadillo is very different: it used templates only, no linking.  RcppGSL
uses GSL; for that we standard configure and system GSL libraries.  None of
these package take anything from another R package, besides Rcpp.

So as a simpler step I would just include package A's code in package B, or
make it one package.

Sorry,  Dirk

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