Re: [boost] py_cpp maintenance release

From: David Abrahams (abrahams_at_[hidden])
Date: 2000-11-04 04:42:10

----- Original Message -----
From: "Prabhu Ramachandran" <prabhu_at_[hidden]>

> >>>>> "David" == David Abrahams <abrahams_at_[hidden]> writes:
>
>     >> 1) The type unsigned char is not wrapped.  So things like this
>     >> dont work.
>     >>
>     >> unsigned char GetDebug();
>
>     David> True. What is the right Python representation for unsigned
>     David> char? I suppose an int would fine. Unless you disagree with
>     David> this I will add the appropriate from_python()/to_python()
>     David> functions for unsigned char to py.h
>
> I guess this should do.  What are the lines I need to add to py.h?

Just copy the corresponding from_python/to_python functions for unsigned
short from py.cpp and py.h, replacing the string "short" with "char". Or you
can try grabbing

http://cvs.sourceforge.net/cgi-bin/cvsweb.cgi/boost/development/py_cpp/py.cp
p?cvsroot=boost

and

http://cvs.sourceforge.net/cgi-bin/cvsweb.cgi/boost/development/py_cpp/py.h?
cvsroot=boost

>     >> 2) The class I am wrapping is called vtkObject.  It is a base
>     >> class from which every other class inherits.  There are a few
>     >> methods in this class that require one to pass a vtkObject
>     >> *obj.  These do not work.  I do not know the best approach to
>     >> solve this problem.  The objects passed are not arrays just
>     >> plain pointers to the base object.  What do I do?  Typical
>     >> methods are
>     >>
>     >> static vtkObject *New() {return new vtkObject;};
>
>     David> I am not surprised you are having trouble with this one.
>
> Yes, so how do I wrap it??  This is a very important function. :)

Have you been watching the boost list? I have answered this question in
several ways over the past few days in the following messages:

http://www.egroups.com/message/boost/6212
http://www.egroups.com/message/boost/6196

 I don't know how to give you more information than that... well, on second
thought maybe in this case I do. It looks to me like this function is
passing back ownership of the vtkObject to the caller. The semantics of
ownership are not implicit in a raw pointer, but they /are/ in a
std::auto_ptr<vtkObject>. If you expose this wrapper function instead, I
think you'll get what you're looking for:

#include <memory>
static std::auto_ptr<vtkObject> vtkObject_New() {
    return std::auto_ptr<vtkObject>(vtkObject::New());
}

> My bad!  Sorry, I cut and pasted the wrong function.  The function
>
>   virtual void UnRegister(vtkObject* o);
>
> needs to be wrapped and hence is in the vtkObject_cb (callback struct)
> and defining it there creates problems because vtkObject* is not yet
> wrapped...  What do you suggest I do?

One way to deal with this is covered in detail in
http://www.egroups.com/message/boost/6224

>     David> Umm... OK, I'm not surprised. If vtkObject has virtual
>     David> functions with default implementations, we are creating a
>     David> "phantom" base class. Try
>     David> dir(vtkObject.__bases__[0]). Ullrich and I are currently
>     David> discussing a way to avoid this phantom base class so that
>     David> dir(vtkObject) has all the methods you expect to see.
>
> Yup, that works.  The phantom base class shows the methods.  Besides,
> calling any static methods of vtkObject give errors because "self" is
> passed to the static c++ function when none is expected.  Therefore
> something like this happens for a static func.
>   static int  GetGlobalWarningDisplay();
> In python
>   >>> a = vtkObject ()
>   >>> a.GetGlobalWarningDisplay ()
>   Traceback (innermost last):
>     File "<stdin>", line 1, in ?
>   TypeError: function requires exactly 0 arguments; 1 given
> # the following works though
>   >>> vtkObject.GetGlobalWarningDisplay ()
>   1

This is normal for Python. Unlike in C++, you can't call a static member
function through an instance. Qualification with an instance causes implicit
binding of the first argument.

> Here is a summary of what I'd like some help with.
>
>   1) What are the functions I need to add to handle the unsigned
>   char.

answered
>   2) How do I wrap the c++ function
>
>   static vtkObject *New() {return new vtkObject;};

answered

>   3) How does one wrap a virtual function that returns/requires a
>   pointer to an instance of the class being wrapped.  Like
>
>   virtual void UnRegister(vtkObject* o);

Good question. It will go in the docs.
The answer is to use explicit instantiation as described in
http://www.egroups.com/message/boost/6224

>   4) A fix for the phantom class would be useful.

I have done that and checked it into CVS

>  It would be nice to
>   be able to call the static methods just like any non static methods.

That's a separate issue, and one that I don't expect to to fix. It's just
not "pythonesque" to do things that way.

-Dave