[C++-sig] extractor

[David Abrahams]

"Mike Rovner" <mike at bindkey.com> writes:

> I'm trying to implement an extractor.
> Unfortunately I couldn't find an example of it.
> Please point me to a simple one.
>
> My code:
> #include <string>
> #include <vector>
> #include <boost/python.hpp>
> using namespace boost::python;
>
> template <class T>
> struct vec_extractor
> {
>   static std::vector<T>& execute(PyObject* obj)
>   {
>     int size=PySequence_Size(obj);
>     std::vector<T> *vec=new std::vector<T>(size);
>     for(int i=0; i<size; ++i)
>       (*vec)[i]=extract<T>(PySequence_GetItem(obj,i));
>     return *vec;
>   }
> };
>
> static std::string test_ext(std::vector<bool>& v)
> {
>   std::string cout("[");
>   for(int i=0; i<v.size(); ++i)
>     cout += (v[i]? 'T': 'F') << ' ';
>   cout += ']';
>   //delete &v;
>   return cout;
> }
>
> BOOST_PYTHON_MODULE(ext)
> {
>   lvalue_from_pytype<vec_extractor<bool>,&PyTuple_Type>();
>   lvalue_from_pytype<vec_extractor<bool>,&PyList_Type>();
>
>   def("test", test_ext);
> }

Mike, 

In this case your source Python object does not contain a vector, so
you should not expect to be able to extract a vector lvalue.  You need
an rvalue from-python converter.  Ralf' solutions in the FAQ are what
you want:

http://cvs.sourceforge.net/cgi-bin/viewcvs.cgi/*checkout*/boost/boost/libs/python/doc/v2/faq.html#question2

-- 
                       David Abrahams
   dave at boost-consulting.com * http://www.boost-consulting.com
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