Re: PyCXX and new style classes

[Murali D. <don...@ya...>]

I also added a constructor in my class B:
B( const WrappedClass* x )  :  m_wrapped_obj(*x) {}


Murali



________________________________
From: Murali Donthireddy <don...@ya...>
To: Barry Scott <ba...@ba...>
Cc: PyCXX and improvement <cxx...@li...>
Sent: Thu, October 28, 2010 7:24:18 PM
Subject: Re: PyCXX and new style classes


Barry,

Thanks for the reply.

BTW, I am using Python2 parts of version 2.6.1 of PyCXX.

I used simple.cxx as my starting point. But all of the methods in 
"new_style_class" return Py::None. How do I create and return an instance of 
new_style_class to the python code?  (In my application, I have more than one 
class, and a method of one class needs to return an instance of another class.)

With some help from someone who knows c++ better than me, I managed to do what I 
needed and it seems to work. But it felt like a herculean effort.

I added a second constructor to PythonClass and modified its self() to take an 
optional "owned" flag. I give all the code further down. 


Let us call my subclasses of PythonClass A and B, they each wrap a pre-existing 
C++ class and hold a shared pointer to instances of those wrapped classes. Let 
us call the pointers a and b.

I am now able to return  a  PythonClassInstance of B from a method A as follows:
    const WrappedClass* x =  < something  that returns an instance of 
WrappedClass>;
    return (new B(x))->self(true);

I must be being monumentally stupid because there is a vast gap between what I 
had to do and your comment "simple.cxx is all you need". I don't see how that 
can be the case when simple.cxx doesn't have a single method that returns 
anything, let alone instances of user defined classes.

Regardless of whether I did more than what was necessary, my extension works 
beautifully. I think PyCXX is a very nice tool. The fact that I could make 
changes to it is an indication of how well written it is. I am grateful for your 
work on PyCXX.

thanks,
Murali

My changes to PythonClass, in header file CXX/Python2/ExtensionType.hxx:

    template<TEMPLATE_TYPENAME T> class PythonClass
    : public PythonExtensionBase
    {
    protected:
[...]
        static PythonClassInstance* alloc_empty() {
            PyObject *py_optlib_expr = extension_object_new(type_object(), 
Tuple().ptr(), Dict().ptr());
            return reinterpret_cast<PythonClassInstance*>(py_optlib_expr);
        }

        PythonClass()
            :  PythonExtensionBase()
            , m_class_instance( alloc_empty() )
        {
            reinterpret_cast< Py::PythonClassInstance * 
>(selfPtr())->m_pycxx_object = 

                static_cast<PythonExtensionBase*>(this);
        }
public:
        static const T* obj_from_arg(Object arg) {
            PythonClassInstance * b = reinterpret_cast<PythonClassInstance *> 
(arg.ptr());
            const T *val = static_cast<T  *>(b->m_pycxx_object);
            return val;
        }
        
        virtual Object self(bool owned = false) // customization: Add optional 
argument owned.
        {
            return Object( reinterpret_cast<PyObject *>( m_class_instance ), 
owned );
        }
[...]


Murali




________________________________
From: Barry Scott <ba...@ba...>
To: Murali Donthireddy <don...@ya...>
Cc: PyCXX and improvement <cxx...@li...>
Sent: Thu, October 28, 2010 4:59:03 PM
Subject: Re: PyCXX and new style classes



On 20 Oct 2010, at 19:14, Murali Donthireddy wrote:

Hi Barry,
>
>I apologize for directly emailing you with a question. Completely understandable 
>if you don't reply.
>


We have a user list that I'm CC'ing.


>I am unable to figure out how to create, in C++ code, an instance of a new style 
>class. The constructor requires a mysterious Py::PythonClassInstance* that I 
>don't know how to generate.
>

You do not call the c'tor directly to create a instance.
You have to create instances using python methods.


>The example simple.cxx seems to be the only source of info on about new style 
>classes anywhere on the Internet, short of reading the implementation of PyCXX, 
>but my C++ isn't good enough for that. 
>

Have you tried compiling and running simple? It shows the new style class stuff 
all working.


>Any pointer or example code would be great.
>

simple.cxx is all you need. It shows you how to do most of the interesting 
stuff.

What exactly are you trying to do that simple.cxx does not do?

Barry