Re: [Swig-user] SWIG, c++ to Python: array of pointers (double pointer) not working

[Nitro <ni...@dr...>]

Am 21.03.2009, 19:13 Uhr, schrieb Josh Cherry <jc...@nc...>:

> Or use something like $descriptor(T *).
>
> However, it seems to me that calling SWIG_ConvertPtr is the wrong thing  
> to
> do.  The object that you get from the list (o) is not a SWIG
> representation of an int*.  Rather, it is a Python integer.  You would  
> use
> something like PyInt_AsLong to get the integer, and PyInt_Check to check
> it.  You would need to store those integers somewhere and create an array
> of pointers to them.  You may need to call Py_DECREF on o to avoid a
> memory leak.

Even simpler, you can use the traits fragment stuff (which is part of the  
UTL I think) and just use the swig::from or swig::from_ptr. Because of  
partial template specialization it will automatically resolve to the  
fitting code ( be it PyInt_AsLong if T is int or if T is an  
std::vector<T2> it will automatically place the code to covert from a  
python sequence to an std::vector<T2> ).
Actually this is a mechanism I'd love to see manifested and used a bit  
more in SWIG, because it's very useful and you don't have to whip up  
custom and redundant typemap code. The swig::from, swig::from_ptr,  
swig::as and swig::as_ptr templates are single entry points to do  
conversion between C++ and target language (e.g. Python-to-C++ or  
C++-to-Python). One place where I use this is to write generic OUTPUT&  
typemaps (e.g. wrapping a function with a prototype like "void  
generateVector( std::vector<int>& vec )" ).

> It would be easier to add a function to the interface that takes an int*
> or, as I suggested earlier, a const vector<int>&.

Yes, that's usually the easiest way. If you sequences are very large, I  
recommend to take a look at numpy and the numpy.i interface file which  
deals with lots of this stuff in a performant, somewhat standard manner.

-Matthias