Raising exceptions in Rcpp - c++

Question

I am trying to report errors from my rcpp code. I am using the constructor exception (const char *message_, const char *file, int line) from http://dirk.eddelbuettel.com/code/rcpp/html/classRcpp_1_1exception.html. To isolate the problem, I wrote the following bar.cpp:
#include <Rcpp.h>
RcppExport SEXP bar( SEXP x){
throw(Rcpp::exception("My Error Message","bar.cpp",4));
return x ;
}
When I run it in R, this is what I get:
> dyn.load("bar.so")
> is.loaded("bar")
[1] TRUE
> .Call("bar",12)
Error: SET_VECTOR_ELT() can only be applied to a 'list', not a 'NULL'
>

Answer 1

You can either
use the inline package which puts a try/catch block into the function it generates for you (by using two simple macros)
or do it manually yourself as shown in a bunch of examples on my blog, or in the examples/ section of the Rcpp package,
but doing what you (ie: throwing outside of a try/catch block) can never work.
As an added bonus, here is a complete example (which essentially already existed in the unit tests):
R> library(inline)
R> f <- cxxfunction(signature(), plugin="Rcpp", body='
+ throw std::range_error("boom");
+ return R_NilValue;
+ ')
R> f()
Error in f() : boom
R>
Again, cxxfunction() puts a try/catch() block in here for you as you can see if you turn verbose on:
R> f <- cxxfunction(signature(), plugin="Rcpp", body='
+ throw std::range_error("boom");
+ return R_NilValue;
+ ', verbose=TRUE)
>> setting environment variables:
PKG_LIBS = -L/usr/local/lib/R/site-library/Rcpp/lib \
-lRcpp -Wl,-rpath,/usr/local/lib/R/site-library/Rcpp/lib
>> LinkingTo : Rcpp
CLINK_CPPFLAGS = -I"/usr/local/lib/R/site-library/Rcpp/include"
>> Program source :
1 :
2 : // includes from the plugin
3 :
4 : #include <Rcpp.h>
5 :
6 :
7 : #ifndef BEGIN_RCPP
8 : #define BEGIN_RCPP
9 : #endif
10 :
11 : #ifndef END_RCPP
12 : #define END_RCPP
13 : #endif
14 :
15 : using namespace Rcpp;
16 :
17 :
18 :
19 : // user includes
20 :
21 :
22 : // declarations
23 : extern "C" {
24 : SEXP file4cc53282( ) ;
25 : }
26 :
27 : // definition
28 :
29 : SEXP file4cc53282( ){
30 : BEGIN_RCPP
31 :
32 : throw std::range_error("boom");
33 : return R_NilValue;
34 :
35 : END_RCPP
36 : }
37 :
38 :
Compilation argument:
/usr/lib/R/bin/R CMD SHLIB file4cc53282.cpp 2> file4cc53282.cpp.err.txt
ccache g++-4.6 -I/usr/share/R/include \
-I"/usr/local/lib/R/site-library/Rcpp/include" \
-fpic -g0 -O3 -Wall -pipe -Wno-unused -pedantic -c file4cc53282.cpp \
-o file4cc53282.o
g++ -shared -o file4cc53282.so file4cc53282.o \
-L/usr/local/lib/R/site-library/Rcpp/lib \
-lRcpp -Wl,-rpath,/usr/local/lib/R/site-library/Rcpp/lib \
-L/usr/lib/R/lib -lR
R>
The BEGIN_RCPP and END_CPP add the magic you need here.
Please do move your questions to rcpp-devel.

Answer 2

Just wrap your code inside BEGIN_RCPP/END_RCPP:
RcppExport SEXP bar( SEXP x){
BEGIN_RCPP
throw(Rcpp::exception("My Error Message","bar.cpp",4));
return x ;
END_RCPP
}
Note that you can throw normal std exceptions too:
throw std::invalid_argument("'x' is too short");