Re: Vptr

From:

"Bo Persson" <bop@gmb.dk>

Newsgroups:

comp.lang.c++.moderated

Date:

Thu, 12 Aug 2010 01:00:08 CST

Message-ID:

<8cg3q1Fs83U1@mid.individual.net>

MC wrote:

Hi!
I have another naive question. I have been trying to understand the
polymorphysim implementation in C++.
I believe that most of the compilers implement it using vptr and
vtables.
My question is when for every object of a class (which has a virtual
member) if vptr points to the same vtable, why does every object
has a copy of the same vptr?

For example
say X is a class which has a virtual function
and a and b are objects of that class
both a and b will have a vptr installed in them and this vptr will
point to the same vtable
Why not have the vptr as a static data member of the class X so that
every object of that class can use it.

I am sure there is a very good reason for the way it is implemented,
but because of my inexperience with C++ I dont see the reason. Can
anybody please elaborate on the reason.

The vtable is interesting only when you have other classes, like Y and
Z, derivning from X. Then a pointer to X could in fact point to an
object of either X, Y, or Z. In that case, the vtable will be used to
select the proper virtual function to call.

If you only have X objects, many compiler will bypass the vtable and
call X functions directly.

Bo Persson

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