Re: [Boost-users] [Boost.Python] Infinite recursion attempting to_python conversion

This post belongs on the C++-sig; followups directed thence. "Nat Goodspeed" writes:

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();

However,

implicitly_convertible();

does NOT allow the produceWString() function to succeed: I still get the aforementioned TypeError.

Right. implicitly_convertible is only used when converting from Python to C++.

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();

Good.

The result is a stack overflow.

Bad.

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().

It's hard to imagine; as you can see cbstring_to_python::convert just calls Python's routine to create a new string. There's no reason for that function to call back into your code. I'm guessing you are blowing the stack somewhere else in your code. I suggest you try to use a debugger and try to track down the real cause of the error. If you can't do that, I suggest you reduce your example to a *minimal* but complete test case. If the answer doesn't jump out at you once you've done that, post it on the C++-sig and someone (maybe me) can try to take a look at it.

How *should* I go about returning our specialized string type as a Python Unicode string?

Your basic approach looks fine. -- Dave Abrahams Boost Consulting www.boost-consulting.com