[C++-sig] Re: Deriving classes

[David Abrahams]

yakumoklesk at yahoo.es writes:

> It is possible to derive a C++ class from a python class? 

Not really.

> The dynamism of python respect the static methods of C++ for
> declaring classes makes me think it won't be possible.
>
> Anyway, what I want is to derive from de top class object from
> python, to take advantage of the use of slots. 

Boost.Python extension classes *are* derived from object, so you
*should* be able to use slots with Python classes derived from
Boost.Python C++ classes...

...argh!  That won't work because of the fact that we're using the
variable-length part of the Python object to help us store C++ objects
directly in the Python instance instead of using a separate dynamic
allocation.  Python won't let you add slots to subclasses of types
with a tp_itemsize.  Well, this is just another piece of evidence
that in-place storage was a poorly-chosen optimzation.  One day we're
going to remove it and you'll be able to add slots in subclasses.

> Can I make it using boost.python or had I to derive from the C++
> class in the python source code?

Well, what you can do is something like the following:

void method1(object self, int x);
std::string method2(object self, char const*);

object make_derived_class(object base_class)
{
    // get the metaclass object
    object metaclass = base_class.attr("__class__");
    
    // populate method dict
    dict d;
    d["method1"] = method1;
    d["method2"] = method2;

    // call the metaclass to produce a derived class
    return metaclass("derived", make_tuple(base_class), d);
}

If you wrap make_derived_class you can then do:

   >>> DerivedWithSlots = make_derived_class(MyClassWithSlots)

Now you have a new Python class with some wrapped C++ functions for
methods.

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