[C++-sig] pyste and virtual classes.

[David Abrahams]

Giulio Eulisse <giulio.eulisse at cern.ch> writes:

>> I can understand that, but I am having a hard time understanding what
>> the use-case for such an arrangement is.  The C++ class designer
>> explicitly said "you can't instantiate a class unless you implement
>> this function".  Now you want to subvert that, in some sense.  Is the
>> result really useful?
>
> I understand what you mean and I would generally agree with what you
> say.
> The point is that in my case I cannot modify the library in question
> (QT) and at the same time I would like to have a working (although very
> limited) python module to see if what I want to do works or not.
> Because of the complexity of QT I have to remove some pure virtual
> methods because otherwise I would end up implementing bindings for the
> whole library.

OK.

> So I'm not saying that it would be really useful or a good coding
> practice but, at least for me, it would be nice to have, for prototyping
> and experimenting, just like the "exclude all that contains xyz"
> feature.

I understand what you're asking for, but I can't for the life of me
understand why you need it.  See ** below.

> Another possible solution would be get rid of the whole boost/QT idea,
> and make PyQT and boost play nicely together

It's not clear that PyQT (SIP) will automatically generate empty
virtual functions for you either.

> (and play nicely with an existing QApplication object). The problem
> is that I have to do the following:
>
> I have a virtual class "Base", 

You must mean an abstract class.  There's no such thing as a virtual
class.

> which derives from a QT one, this class gets derived into "Derived"
> and extended in python. An instance of the class "Derived" is
> created in an embedded python environment and is passed to C++ using
> something like extract<Base&>.

It's not clear what you mean by "passed to C++ using
extract<Base&>".  You can only write extract<Base&> in C++, so the
object must already be available in C++ code somehow.

>  The virtual method MyBase.method() is then called in C++ and its
> python implementation in Derived gets called.

**OK so far; I don't see any need to provide a C++ implementation of
the virtual member function other than the one provided automatically
by pyste's generated wrapper class.  That alone should be enough to
ensure that there is no problem with instantiating an abstract base
class.

> Both the C++ and the Python environment need to access the
> QT part of the class, and I don't want (better: I have not to) to
> write wrappers around all the QT functions I need. I haven't figured
> out how to do the extract<Base&> part when using PyQt. Maybe you can
> help me?

I don't know the first thing about PyQt, but the real issue is that I
can't understand the problem you're having.

-- 
Dave Abrahams
Boost Consulting
www.boost-consulting.com