[Rcpp-devel] experimental handling of uncaught exceptions

[Romain François]

On 12/27/2009 09:03 PM, Dirk Eddelbuettel wrote:
>
> On 27 December 2009 at 13:36, Dirk Eddelbuettel wrote:
> | On 27 December 2009 at 18:15, Romain François wrote:
> | | Hi,
> | |
> | | I've commited some code that aims at forwarding uncaught C++ exceptions
> | | to R.
> | |
> | | For example :
> | |
> | | require( Rcpp)
> | | funx<- cfunction(signature(), '
> | | throw std::range_error("boom") ;
> | | return R_NilValue ;
> | | ', Rcpp=TRUE, verbose=FALSE)
> | | tryCatch(  funx(), "C++Error" = function(e){
> | | 	print( "gotcha" )
> | | } )
> | | print( rnorm(10) )
> |
> | That's pretty crazy cool -- I'll have to try that.
>
> Can we get access to the string 'boom' at the R level?

I am not sure. I think we cannot. I would also like to reach the class 
of the exception "range_error" to bring direct handlers (once again the 
idea is rJava inspired) but this does not seem possible.

If C++ had reflection, then this would be possible;

tryCatch( funx(), range_error = function(e){
	# do something with the range error exception
} )

This does not aim to replace explicit try/catch within the code, but it 
prevents the R session from crashing if some exception is not caught.

> That's how the
> existing C++-based try/catch ->  Rf_error() works.

For this I thought that maybe we could have an "r_error" exception, with 
an overloaded throw operator so that we would do :

throw r_error( "boom" ) ;

> Also, the onAttach trick won't work for other packages just including Rcpp.h
> et al and linking to libRcpp.{so,a}.

These packages can just call "initUncaughtExceptionHandler" from their 
C++ side, this is the only thing the .onAttach does.

Now the mechanism calls back an R function that lives (and is not 
exported from) the Rcpp namespace.

uncaught_cpp_exception <- function( message = "uncaught C++ exception" ){
	callstack <- sys.calls()
	ncalls <- length(callstack)
	call <- if( ncalls > 1L) callstack[[ ncalls - 1L ]] else match.call()
	classes <- c( "C++Error", "error", "condition" )
	condition <- structure(
		list( message = message, call = call ),
		class = classes )
	stop( condition )
}

Are there cases where the libRcpp.{so,a} is used but the Rcpp package is 
not available ?

We could have it all in C, but this leads to more complicated code. See 
the file rJava.c attached for how Simon does it in rJava. (look between 
hack and hack)

> Dirk
>


-- 
Romain Francois
Professional R Enthusiast
+33(0) 6 28 91 30 30
http://romainfrancois.blog.free.fr
|- http://tr.im/IlMh : CPP package: exposing C++ objects
|- http://tr.im/HlX9 : new package : bibtex
`- http://tr.im/Gq7i : ohloh

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