Question

I have my own string class and I want to export it into python (with boost.python) and use as a native string. I wrote converters for this.

The first is exporting of my string:

bp::class_<CL_StringRef8>("CL_StringRef8", bp::init<const std::string&>())
    .def("CStr", &CL_StringRef8::c_str);

Converters are from default tutorial but with my type:

// CL_StringRef8 → Python string --------------------------------------
struct cl_stringref8_to_python_str
{
    static PyObject* convert(CL_StringRef8 const& s)
    {
        return boost::python::incref(boost::python::object(s.c_str()).ptr());
    }
};

// Python string → CL_StringRef8 --------------------------------------
struct cl_stringref8_from_python_str
{
    cl_stringref8_from_python_str()
    {
        bp::converter::registry::push_back(
            &convertible,
            &construct,
            boost::python::type_id<CL_StringRef8>()
        );
    }

    static void* convertible(PyObject* obj_ptr)
    {
        if (!PyString_Check(obj_ptr)) return 0;
        return obj_ptr;
    }

    static void construct(PyObject* obj_ptr, bp::converter::rvalue_from_python_stage1_data* data)
    {
        const char* value = PyString_AsString(obj_ptr);
        if (value == 0) bp::throw_error_already_set();
        void* storage = ((bp::converter::rvalue_from_python_storage<CL_StringRef8>*)data)->storage.bytes;
        new (storage) CL_StringRef8(value);
        data->convertible = storage;
    }
};

Now, as I understand, I can call c++ functions from python which takes in arguments CL_StringRef8, but I have a code:

print GetMyStringObject()
data = float(GetMyStringObject())

# ==>

<CL_StringRef object at 0x7fd15dfd9de8>
TypeError: float() argument must be a string or a number

As I understand I have to export str() method for the CL_StringRef8 but I can’t do that:

bp::class_<CL_StringRef8>("CL_StringRef8", bp::init<const std::string&>())
    .def("CStr", &CL_StringRef8::c_str)
    .def(bp::self_ns::str(bp::self_ns::self)); 

Last line calls an error:

/usr/include/boost/lexical_cast.hpp: In member function ‘bool boost::detail::lexical_stream<Target, Source, Traits>::operator<<(const Source&) [with Target = std::basic_string<char>, Source = CL_StringRef8, Traits = std::char_traits<char>]’:
/usr/include/boost/lexical_cast.hpp:1151:13:   instantiated from ‘Target boost::detail::lexical_cast(typename boost::call_traits<Source>::param_type, CharT*, std::size_t) [with Target = std::basic_string<char>, Source = CL_StringRef8, bool Unlimited = true, CharT = char, typename boost::call_traits<Source>::param_type = const CL_StringRef8&, std::size_t = long unsigned int]’
/usr/include/boost/lexical_cast.hpp:1174:77:   instantiated from ‘Target boost::lexical_cast(const Source&) [with Target = std::basic_string<char>, Source = CL_StringRef8]’
/usr/include/boost/python/operators.hpp:357:1:   instantiated from ‘static PyObject* boost::python::detail::operator_1<(boost::python::detail::operator_id)19u>::apply<T>::execute(boost::python::detail::operator_1<(boost::python::detail::operator_id)19u>::apply<T>::self_t&) [with T = CL_StringRef8, PyObject = _object, boost::python::detail::operator_1<(boost::python::detail::operator_id)19u>::apply<T>::self_t = CL_StringRef8]’
/usr/include/boost/python/operators.hpp:152:11:   instantiated from ‘void boost::python::detail::operator_<id, L, R>::visit(ClassT&) const [with ClassT = boost::python::class_<CL_StringRef8>, boost::python::detail::operator_id id = (boost::python::detail::operator_id)19u, L = boost::python::detail::not_specified, R = boost::python::detail::not_specified]’
/usr/include/boost/python/def_visitor.hpp:31:9:   instantiated from ‘static void boost::python::def_visitor_access::visit(const V&, classT&) [with V = boost::python::def_visitor<boost::python::detail::operator_<(boost::python::detail::operator_id)19u> >, classT = boost::python::class_<CL_StringRef8>]’
/usr/include/boost/python/def_visitor.hpp:67:9:   instantiated from ‘void boost::python::def_visitor<DerivedVisitor>::visit(classT&) const [with classT = boost::python::class_<CL_StringRef8>, DerivedVisitor = boost::python::detail::operator_<(boost::python::detail::operator_id)19u>]’
/usr/include/boost/python/class.hpp:225:9:   instantiated from ‘boost::python::class_<T, X1, X2, X3>::self& boost::python::class_<T, X1, X2, X3>::def(const boost::python::def_visitor<Derived>&) [with Derived = boost::python::detail::operator_<(boost::python::detail::operator_id)19u>, W = CL_StringRef8, X1 = boost::python::detail::not_specified, X2 = boost::python::detail::not_specified, X3 = boost::python::detail::not_specified, boost::python::class_<T, X1, X2, X3>::self = boost::python::class_<CL_StringRef8>]’
/home/ockonal/Workspace/Themisto/src/Scripts/Core/TypesConverters.cpp:375:49:   instantiated from here
/usr/include/boost/lexical_cast.hpp:595:48: error: cannot bind ‘std::basic_ostream<char>’ lvalue to ‘std::basic_ostream<char>&&’
/usr/lib/gcc/x86_64-unknown-linux-gnu/4.6.0/../../../../include/c++/4.6.0/ostream:581:5: error:   initializing argument 1 of ‘std::basic_ostream<_CharT, _Traits>& std::operator<<(std::basic_ostream<_CharT, _Traits>&&, const _Tp&) [with _CharT = char, _Traits = std::char_traits<char>, _Tp = CL_StringRef8]’

---

ps and yeah, I didn’t forget to register my converters into python.

Answer 1

In order for the .def(bp::self_ns::str(bp::self_ns::self)); line to work, you need to have the streaming operator defined for your class.

std::ostream & operator<<(std::ostream & os, const CL_StringRef8 &s)
{ os << s.c_str(); return os; } 

An alternative way of defining the str() method without needing the streaming operator is:

.def("__str__", &CL_StringRef8::c_str) 

Although, I prefer to just do the repr() method instead which is then used by str() when needed

.def("__repr__", &CL_StringRef8::c_str)  

All this still won’t make float() work in Python, since it expects either a string (not an object, even if it supports __str__), a float, or an object that supports __float__.

We can do the latter though, by binding a lexical_cast around your c_str() method:

#include <boost/lexical_cast.hpp>
#include <boost/mpl/vector.hpp>  

and then

.def("__float__", bp::make_function( 
                    boost::bind(&boost::lexical_cast<float,CL_StringRef8>,_1),
                      bp::default_call_policies(), 
                        boost::mpl::vector<float,CL_StringRef8>()));

Now you can do this in Python:

>>> a = CL_StringRef8("1.234")
>>> print a
1.234
>>> float(a) * 2
2.468