[C++-sig] Strange boost python error message (and solution)

[François Duranleau]

On Thu, 18 May 2006, Allen Bierbaum wrote:

> I just ran into a very strange error message in boost python that took
> me all afternoon to track down.  I thought I would post the error and
> the solution here to:
>
>      a) help other people out if they run into the same problem
>      b) see if anyone can tell me what is really going on behind the
> scenes and if there is a way for boost python to detect it
>
> I was getting an assertion in boost::python::converter::registery::insert
>
> python: boost/libs/python/build/../src/converter/registry.cpp:162:
> void boost::python::converter::registry::insert(PyObject* (*)(const
> void*), boost::python::type_info): Assertion `slot == 0' failed.
>
> I tracked it down to having two calls to
> boost::python::register_ptr_to_python< boost::shared_ptr< MyClass > >
> .  As long as there is only one call like this in the module for a
> single class things are fine.  The second call causes a problem.
>
> So can anyone that knows the details of boost.python internals explain
> what is happening here?
>
> My guess (and it is only a guess) is that the registration of  a
> to_python conversion function is detecting that it has already been
> set and asserting.

As far as I can tell from my understanding of the source code, this is 
right.

>  If this is true, could the assertion be changed to
> something a little more descriptive like:
>
> assert((slot == 0) && "Attempted to register a to_python conversion
> function when one is already registered");

Sounds good. But also, would it be possible to detect if the same 
to_python converter is get registered, and in such a case simply do 
nothing and thus report no error?

-- 
François Duranleau
LIGUM, Université de Montréal

"Il n'existe ni caprices du sort, ni bizarreries, ni accidents. Il n'y a
  que des choses que les humains ne comprennent pas. [...] Les humains ne
  peuvent exister si tout ce qui est déplaisant est écarté au lieu d'être
  compris."
             - Cygne Royal, dans _Message des hommes vrais au monde mutant_
                                                          (de Marlo Morgan)