For some purposes we have a special string class derived from std::wstring. Simplified declaration: class cbstring: public std::wstring { public: cbstring(const wchar_t* that): std::wstring(that) {} cbstring(const std::wstring& that): std::wstring(that) {} ... }; I want to return such objects from C++ as Python Unicode strings, e.g.: cbstring produceWString() { return L"This is a test Unicode string"; } If produceWString() returns std::wstring, the conversion happens by magic. But with the cbstring return type, I get: TypeError: No to_python (by-value) converter found for C++ type: class cbstring The corresponding conversion problem (passing u"Foo" from Python to a function accepting const cbstring&) can be handled with the following call in the extension module: implicitly_convertible<std::wstring, cbstring>(); However, implicitly_convertible<cbstring, std::wstring>(); does NOT allow the produceWString() function to succeed: I still get the aforementioned TypeError. So I declared a type converter: struct cbstring_to_python { static PyObject* convert(const cbstring& cbs) { return PyUnicode_FromUnicode(cbs.c_str(), cbs.length()); } }; and registered it with: to_python_converter<cbstring, cbstring_to_python>(); The result is a stack overflow. I'm not clear on exactly why it happens, but something inside the cbstring_to_python converter is apparently recursively invoking cbstring_to_python::convert(). How *should* I go about returning our specialized string type as a Python Unicode string? OS: Microsoft Windows XP Pro, SP1 Compiler: Microsoft VC++ 7.1 with STLport 4.6 Boost version: 1.31