Re: call base class function or derived class function

From:
Abhishek Padmanabh <abhishek.padmanabh@gmail.com>
Newsgroups:
microsoft.public.vc.language
Date:
Tue, 18 Mar 2008 08:49:19 -0700 (PDT)
Message-ID:
<e12f1a16-fdab-4e43-913a-2c591bed9cb2@s12g2000prg.googlegroups.com>
On Mar 18, 8:19 pm, "Ben Voigt [C++ MVP]" <r...@nospam.nospam> wrote:

Abhishek Padmanabh wrote:

"George" <Geo...@discussions.microsoft.com> wrote in message
news:74F39427-6E0E-46CA-9D92-EF3C79D99651@microsoft.com...

Thanks Igor,

You could a) look at vtable under debugger, or b) try to call the


I doubt whether method (a) works. You know we can not create an
object of abstract class, and we can only create an instance of
derived class. How could we monitor the vtable of the abstract base
class to see whether the pure virtual method of base class is in
vtable of instance of abstract base
class?

And if we can create an instance of derived class, it means it
implements the pure virtual method, then in the vtable of instance
of derived class, it
should be the virtual method of derived class, not the pure virtual
method of
abstract base class.


a) should work. You cannot create an object of abstract class type
but that is not a problem. I might be misreading the debugger but I
think that the abstract class' v-table is not needed and hence not


The compiler cannot avoid generating the abstract class v-table in all
cases. Consider:

During the execution of the abstract base class constructor, the dynamic
type is the abstract class. The exact type is statically known, so when=

 the

constructor calls any member function the call is non-virtual. However,=

 the

constructor may call a member function which in turn calls a virtual
function. This helper function cannot guarantee the dynamic type, becau=

se

it could be called outside the constructor as well. Hence it must call
through the v-table. But the dynamic type of the object is the abstract=

type, the derived class v-table cannot be used. Hence vfptr must point =

to

the abstract class v-table.- Hide quoted text -


Thanks, Ben. I was thinking that the c-tor/d-tors could be exception
cases but was not able to reason why, which you clearly pointed out.

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