unused variable warnings issued by Rcpp

[ablaette]

When compiling RcppCWB you may see the following warning:

/Library/Frameworks/R.framework/Versions/4.1/Resources/library/Rcpp/include/Rcpp/DataFrame.h:136:18: warning: unused variable 'data' [-Wunused-variable]
SEXP data = Parent::get__();
^

This does not result from C/C++ code of RcppCWB, but from the Rcpp package. Thus, compiling the Rcpp version currently available at CRAN (v1.0.6) also yields this warning. On my system:

../inst/include/Rcpp/DataFrame.h:136:18: warning: unused variable 'data' [-Wunused-variable]
SEXP data = Parent::get__();

This is just a warning and does not indicate real trouble. For the time being, the solution is to install an updated version of Rcpp not yet available at CRAN. A conversation on Stackoverflow explains that it is available via a drat repository:

install.packages("Rcpp", repos="https://rcppcore.github.io/drat")

The alterntive to updating Rcpp is to add the flag -Wno-unusedto the CXXFLAGS in ~/.R/Makevars, but this conceals something that can and should be fixed easily. This is why I think that using the updates of Rcpp is the better solution. In my case v1.0.6.7 did the job.

It is just a matter of time that a new Rcpp version is released via CRAN which will not result in the warning. Then this issue can be closed.

[ablaette]

Update: The warning depends on the compiler. I see it with clang v11.0.3 (macOS), but gcc v10.3.0 (Debian). Anyway, as Rcpp v1.0.7 is available as a binary package for all systems on CRAN, requiring this version (in DESCRIPTION, Suggests:) is the solution.