[C++-sig] Re: deriving in python from a C++ base class

[David Abrahams]

Stefan Seefeld <seefeld at sympatico.ca> writes:

> Ralf W. Grosse-Kunstleve wrote:
>> --- Stefan Seefeld <seefeld at sympatico.ca> wrote:
>> 
>>>class MyWorld(World):
>>>     def __init__(self):
>>>         self.set('hi there')
>>>         print self.greet()
>> I think you have to call World.__init__() before you can use the
>> instance.
>> Here is more information:
>> http://www.boost.org/libs/python/doc/PyConDC_2003/bpl.html#inheritance
>
> thanks, that works !
>
> I still have a question, though:
>
> The reason I missed that was that in my real code I initialized the
> class_<> with a no_init argument, as that was declared to be the way
> to make the (C++) class non-instantiable.

Passing no_init to the class_<...> doesn't affect the C++ class'
instantiability; it only affects the instantiability of the
corresponding Python class.

> However, even though I only want subclasses to be instantiated, I of
> course do need to initialize the base class (i.e. run its
> constructor).
>
> How should I do that, then ?

I'm afraid there's no good way to impose that restriction yet.  As a
matter of fact, no_init should be more sophisticated and raise an
exception only when the Python class of the "self" argument is an
exact match for the wrapped class (not a subclass).  Maybe you'd like
to contribute the code for that?

Note, however, that it still wouldn't be a very useful check, since
there's nothing to prevent:

        >>> class Foo(WrappedClass):
        ...    pass
        ...
        >>> x = Foo()

Foo is essentially identical to WrappedClass, so what have you
enforced by preventing:

        >>> x = WrappedClass

??

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