Re: virtual inheritance

From:
James Kanze <james.kanze@gmail.com>
Newsgroups:
comp.lang.c++
Date:
Sun, 13 Apr 2008 08:11:42 -0700 (PDT)
Message-ID:
<02f46fed-29a0-4847-8648-f7c883e59b03@c65g2000hsa.googlegroups.com>
On 13 avr, 15:11, Rahul <sam_...@yahoo.co.in> wrote:

 I have the following class hierarchy,

         A
    B C
         D

 B and C are derived from A and D is derived from B and C. Now in
order to avoid the duplication problem, virtual inheritance can be
used. But i'm not sure as to where it should be used?

 Should B and C inherit A virtually or should D inherit virtually B
and C?


B and C should inherit virtually from A.

I'm confused as i'm dealing with a case where A, B and C are provided
by different vendors. B knows that it has to inherit from A but is not
aware about anything else, so is the case with C.


So they should inherit virtually, unless they need a private
copy of A, which isn't shared. In general, unless you know the
full hierarchy, virtual inheritance should be the default.

Now how does the vendor providing class D solve the
duplication problem?


By getting the vendors of B and C to do things correctly.

Seriously, it can be a problem, because this issue seems to be
frequently ignored. (Logically, one wishes that virtual
inheritance had been the default.)

The first thing you might ask is: does it matter? If A has no
data members (which is most often the case when classes are
inheriting from it), it might not matter. You end up with two A
objects (and two A* for each D object), but it typically doesn't
matter which one gets used.

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