[C++-sig] How do I wrap virtual methods

[David Abrahams]

Eric Jardim <ericjardim at gmail.com> writes:

> 2005/8/5, David Abrahams <dave at boost-consulting.com>:
>> 
>> Does the tutorial really recommend that default_f? I can't imagine
>> why. That's just wrong; you don't need it and it's a bad idea to use
>> it. [Joel, can you please fix this ASAP, and the use of default_f
>> further down?]

My mistake; I was just mis-remembering.  Please see:

   http://mail.python.org/pipermail/c++-sig/2004-July/007844.html

[And my apologies to Joel]

> Beside the tutorial, Pyste itself generate "default_..." methods.

That is correct.

>> Another thing that I noticed is that I have to reimplement virtual methods
>> > on every derived class. This is an accumulative task. Am I right?
>> 
>> I don't know what you mean.
>
>
> Suppose you have class A, B and C. C is derived from B, and B derived from A 
> (A <- B <- C). Suppose you have a virtual method "f" (pure or not) defined 
> on the A class scope. Suppose that you want to expose this method, so user 
> can, in Python, extend some class (A, B or C)
>
> If you want to extend class A, in Python, that's Ok. When you create a new A p
> class in Python, you are actually creating a A_Wrapper object. But if you 
> want to extend B or C, they don't know about the A_Wrapper. So, you must 
> create B_Wrapper, C_Wrapper and re-expose "f" on both.
>
> Hope to make it clear now. Is this right, or not.

If you want to derive from B or C in Python and override "f" such that
when C++ calls it, the Python implementation is invoked, that's
correct.

> I still don't understand: should I use the "defaul_..." function (in a 
> different way) or not use it?

Use it.

> Let's see a real (correct) example, please.

You can always know that the examples in libs/python/test are correct.
See libs/python/test/polymorphism2.cpp/.py

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