[Rcpp-devel] assert() for Rcpp?

Nathan Kurz nate at verse.com
Wed Feb 18 21:18:56 CET 2015

On Tue, Feb 17, 2015 at 4:41 PM, Miratrix, Luke
<lmiratrix at fas.harvard.edu> wrote:
> The proposed code:
> #include <stdio.h>
> #ifdef NDEBUG
> # define assert(EX)
> #else
> # define assert(EX) (void)((EX) || (__assert (#EX, __FILE__, __LINE__),0))
> #endif
> void __assert (const char *msg, const char *file, int line) {
>     char buffer [100];
>     snprintf( buffer, 100, "Assert Failure: %s at %s line #%d", msg, file,
> line );
>     ::Rf_error( buffer );
> }

Getting more people using assert-like macros seems like a great idea.
Weighing in as a C programmer with limited knowledge of Rcpp, I'd

1) Don't redefine assert() or __assert().  You'll confuse people and
it will somehow manage to break things.  Instead, define your own
macro with a different name, likely one in all caps that starts with
"R_" or "RCPP_".

2) Only keep the name 'assert' in the macro if you are keeping the
semantics of assert(), that is, it dies on failure if NDEBUG not
defined.  If it prints a warning and recovers, or defaults to off, use
a different word.   I personally don't like the inverted NDEBUG
approach, so would suggest a different semantics and different word.

3) It's debatable if you want to allow it to be turned off or not.
If it can be turned off, people will misuse it to guard against errors
and be surprised when it doesn't.   I often use a noisy warning that
only runs when DEBUG is positively defined (DEBUG_WARN_IF) and an
abort that  cannot be turned off (ERROR_ABORT_IF).

4) Don't try to snprintf() to a local buffer and then return.   I
don't know C++ semantics for exceptions, but at least in C, combining
local stack variables with stack unwinding is asking for trouble.  If
you are aborting, print to STDERR directly.  If you are continuing,
use a normal heap allocation.

Nathan Kurz
nate at verse.com

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