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

[Nicodemus]

I had checked the FAQ and tried this code before I posted earlier, but 
it gave me the same result (the application locks in a loop in that same 
line). Any other ideas of what maybe wrong?

Thanks for your reply Ralf,
Bruno da Silva de Oliveira.

Ralf W. Grosse-Kunstleve wrote:

>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.
>>    
>>
>
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