[C++-sig] Conversion from python objects to c++ objects for argument passing

[Ralf W. Grosse-Kunstleve]

I don't know what is wrong with your code specifically, but you are using a
lvalue converter where you should be using a rvalue converter anyway. lvalue
converters are suitable only for accessing exisiting objects directly. You are
creating a new object, as a static object. This is not thread-safe and in all
likelihood has other drawbacks.
For more information please refer to the

How can I wrap functions which take C++ containers as arguments?

section at

http://www.boost.org/libs/python/doc/v2/faq.html

You may copy container_conversions.h to your own include tree. Include the file
in your extension module and then:

    scitbx::boost_python::container_conversions::from_python_sequence<
      std::vector<int>,
      scitbx::boost_python::container_conversions::variable_capacity_policy>();

This should be all you need.
Ralf

--- Nicodemus <nicodemus at globalite.com.br> wrote:
> Hail!
> 
> First, let me congratulate everyone involved in Boost.Python. The more 
> you use it, the more you see how great the library is!
> 
> I'm having a little problem though. I want that the following function:
> 
> void PrintVec( const vector<int> &v )
> {
>     vector<int>::const_iterator it = v.begin();
>     stringstream ss;
>     ss << "C++: [";
>     while ( it != v.end() ){
>         if ( it != v.begin() ){
>             ss << ", ";
>         }
>         ss << *it;
>     }
>     ss << "]";
>     cout << ss.str() << endl;
> }
> 
> Be called like this from python:
> 
>  >>> PrintVec([1,2,3])
> C++: [1, 2, 3]
> 
> To accomplish that, I've been using this lvalue converter:
> 
> struct pylist_to_vector
> {
>     static vector<int>& execute(PyObject& o)
>     {
>         static vector<int> v;
>         v.clear();
>         cout << "execute" << endl;
>         if ( PyList_Check(&o) ){           
>             int size = PyList_Size(&o);
>             cout << "size is " << size << endl;
>             v.reserve( size );
>             for( int i = 0; i < size; ++i ){
>                 PyObject* item = PyList_GetItem(&o, i);
>                 if ( PyInt_Check(item) ){
>                     int num = PyInt_AsLong(item);
>                     v.push_back( num );
>                 }
>                 else {
>                     cout << "Not a num at: " << i << endl;
>                 }
>             }
>         }       
>         cout << "returning..." << endl;
>         return v;
>     }
> };
> 
> and in the module definition:
> 
> BOOST_PYTHON_MODULE(test)
> {  
>     lvalue_from_pytype<pylist_to_vector, &PyList_Type>();  
>     def( "PrintVec", &PrintVec );       
> }
> 
> Everything links and compiles without a problem. But, when I execute:
>  >>> PrintVec([1,2,3])
> execute
> size is 3
> returning...
> 
> in python, it enters in an infinite loop and never stops. Debugging, I 
> found out that it enters into my function, exits it, goes through 
> boost.python, until it enters this code:
> 
> /* file returning.hpp, line 114
> template<class P BOOST_PP_ENUM_TRAILING_PARAMS_Z(1, N, class A)>
> static PyObject* call(
>         R (*pf)(BOOST_PP_ENUM_PARAMS_Z(1, N, A))
>         , PyObject* args_
>         , PyObject*, P const* policies)
>     {
>         // check that each of the arguments is convertible
>         BOOST_PP_REPEAT_1ST(N, BOOST_PYTHON_CHECK_CONVERSION, nil)
> 
>         if (!policies->precall(args_))
>             return 0;                                          
>         (*pf)(BOOST_PP_REPEAT_1ST(N, BOOST_PYTHON_CALL_ARGS, nil));
>         return policies->postcall(args_, detail::none());
>     }
> 
> In the "return 0;" line, the processor hits 100% and the application blocks.
> 
> So, what am I doing wrong?  Is this the right way to accomplish what I 
> want? I'm using Intel C++ 6 with STLport on windows.
> 
> Thanks for any hints and comments,
> Bruno da Silva de Oliveira.


__________________________________________________
Do you Yahoo!?
Yahoo! Mail Plus - Powerful. Affordable. Sign up now.
http://mailplus.yahoo.com