[C++-sig] [boost.python] convert swig-wrapped c++-return values to python with boost.python

[Maximilian Matthe]

David Abrahams schrieb:
> 
> 
> -- 
> Dave Abrahams
> Boostpro Computing
> http://boostpro.com
> 
> On Jul 21, 2008, at 4:56 AM, Maximilian Matthe <Maxi.Matthe at web.de> wrote:
> 
>> David Abrahams schrieb:
>>> on Sun Jul 20 2008, Maximilian Matthe <Maxi.Matthe-AT-web.de> wrote:
>>>> David Abrahams schrieb:
>>>>> on Sun Jul 20 2008, Maximilian Matthe <Maxi.Matthe-AT-web.de> wrote:
>>>>>
>>
>>>>>> on Sun Jul 20 2008, Maximilian Matthe <Maxi.Matthe-AT-web.de> wrote:
>>>>>> I dont want to return wxWindow by value, but by reference/adress. 
>>>>>> Thus
>>>>>> the bp::reference_existing_object call policy.
>>> You can't use that policy unless you're returning a Boost.Python-wrapped
>>> class.  Its whole purpose in life is to make an instance of the class
>>> where the C++ object is held by raw pointer.  I suggest you leave it 
>>> out.
>>
>> Uh, well, then this could be written into the documentation, couldn't it?
> 
> Lots more could. Care to submit a patch?
> 
>> Then I wouldn't have searched so long for a solution.
> 
> Sorry
> 
>>
>>> I can't imagine what purpose that const_cast could serve.  Surely
>>> wxPyMake_wxObject doesn't modify its first argument?  I suggest you
>>> leave it out.
>>>
>> No, the const_cast is needed. wxPyMake_wxObject wraps a PyObject 
>> around the passed wxObject (any wxWidgets-Class). This object is then 
>> passed to python (contains a pointer to the actual wx-object), which 
>> can be modified in python, thus the const_cast is needed.
> 
> I can't see the code any more, but it sure looked to me like you were 
> adding or removing const from/to the pointer type rather than the pointee.
Here's the code again:
template <class Ptr>
struct convert_wxObjectPtr
{
   static PyObject* convert(Ptr const& o)
   {
     PyObject* arg = wxPyMake_wxObject(const_cast<Ptr>(o), false);
     if(!arg)
       throw python_error("Object is not convertible to wxPython!");
     return arg;
   }
};
You're right, I tried to remove the const_cast from a converter that 
converted wx-objects instead of pointers to them, which lead to a 
compiler error.


>>
>>> HTH,
>>
>> I have found a solution after a very long search and with help from 
>> the irc-channel: Roman Yakovenko has created the return_value_policy 
>> return_pointee_value. Here's his announcement for that:
>> http://mail.python.org/pipermail/c++-sig/2006-November/011568.html
>> Maybe it would be good to have that in boost-python? He has already 
>> added it to his Py++ code generator.
> 
> A patch with docs and tests would go as long way toward making that happen.
> 
I can try to submit something like that, but I've never done something 
like this before. I'll read the documentation at boost.org.
>>
>> So my problem seems to be solved in this case...
>> Thanks for your help!
> 
> NO
What does that mean? Is this an abbreviation or the word No? Isn't it 
solved?
> 
>> PS: I'm currently reading your MPL-Book and like it very much, 
>> although it's quite complicated to see through all the details after 
>> reading one chapter once :-)
> 
> 
> Glad you're enjoying!