GammaLib

I checked the %factory directive, but this requires that all derived types are known beforehand. This is not practical for our case as we want the library to be extendable, and we don’t want to hard code derived class types into the base class.

Note that in typemaps/swigtype.swg there exists the following typemap which may be useful:

/* DYNAMIC typemap */
%typemap(out,noblock=1) SWIGTYPE *DYNAMIC, SWIGTYPE &DYNAMIC {
%set_output(SWIG_NewPointerObj(%as_voidptr($1), SWIG_TypeDynamicCast($descriptor, %as_voidptrptr(&$1)), $owner | %newpointer_flags));
}

Here a couple of links that may be useful:
http://comments.gmane.org/gmane.comp.programming.swig.devel/21148
http://osdir.com/ml/programming.swig/2003-09/msg00066.html
http://old.nabble.com/Status-of-SWIG_TypeDynamicCast--td24364473.html
http://stackoverflow.com/questions/3921294/how-do-i-down-cast-a-c-object-from-a-python-swig-wrapper
http://altdevblogaday.com/2011/08/10/swig-casting-revisited/
http://osdir.com/ml/programming.swig/2004-08/msg00057.html

An interesting option could be the use of:

obj = SWIG_NewPointerObj(widget, SWIG_TypeQuery(myTypeString), 0)

where myTypeString is a string representation of the type. We could add a generic type method that returns the class type as string, which we then could use for type casting. Maybe this can be done at SWIG interface level only, avoiding to pollute the C++ classes?

Maybe we can add a method to each derived class (here GCTAObservation) of the type:

%{
swig_type_info* swig_type(void) {
return SWIGTYPE_p_GCTAObservation;
}
%}

and then define this as a pure virtual method of the base class. We may then use

obj = SWIG_NewPointerObj(base, base->swig_type(), 0)

so that the object base gets downcasted.

Here a similar code fragment from http://www.dabeaz.com/cgi-bin/wiki.pl?SwigFaqCPlusPlusFactoryIntrospection:

%typemap(out) Base* Base::create {
swig_type_info* info = objectIDToSWIGInfo($1->objectID());
if (!info)
SWIG_exception_fail(SWIG_RuntimeError, "Fatal error: objectIDToSWIGInfo returned null.");
$result = SWIG_NewPointerObj(SWIG_as_voidptr($1), info, SWIG_POINTER_OWN);
};

Here another note on using SWIG_TypeQuery:

Yes that should work. One caveat: SWIG_TypeQuery() is pretty slow. To speed up conversions, it may be better to call it once. For example:

static swig_type_info *mytype_descriptor = 0;
if (!mytype_descriptor) {
mytype_descriptor = SWIG_TypeQuery("mytype *");
}
obj = SWIG_NewPointObj(widget, mytype_descriptor, 0);