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

From: David Abrahams (dave_at_[hidden])
Date: 2005-09-16 05:45:58

This post belongs on the C++-sig; followups directed thence.

"Nat Goodspeed" <ngoodspeed_at_[hidden]> 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<std::wstring, cbstring>();
>
> However,
>
>     implicitly_convertible<cbstring, std::wstring>();
>
> 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<cbstring, cbstring_to_python>();

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