what is vtable exactly?is it a structure or an array?

[ypjofficial]

Hello All,
I have read in many c++ literature that vtable is nothing but an array
of pointer to virtual functions inside a class.And the class where the
virtual function/s are declared stores the vfptr i.e the pointer to
vtable internally.
What confuses me is
if vtable is an array of pointer to virtual functions then as per the
properties of the array all the entries inside the array must be
same.i.e all the pointers should be of similar type.and this will put
restriction on the declaration of the virtual functions as they must
have same singnature as per my point.

eg. class base
{
public:
virtual void fun(int,int);
virutal int fun2(char);
};
now according to the definition of vtable(i.e array of pointers to
virtual functions)
what will be the syntax for the array? as single array can't hold
entries with different datatypes.
the pointer to first virtual function is something like
void (base::*fptr1)( int,int) and for second its like
int (base::*fptr2)( char)
how come a single array like vtable will stored these two totally
different entires?

but a structure can stores mulitple datatypes.
So is Vtable a structure or an array? if its array then can anyone give
me its definition?

Thanks and regards,
Yogesh Joshi
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[JustBoo]

On 4 Jan 2006 09:24:59 -0500, "Meador Inge" <me*****@gmail. com> wrote:

Earl Purple wrote:

yp*********@ind iatimes.com wrote:
The v-table is beyond C++. You cannot access it directly, or get a
pointer to a v-table.

That is not 100% true. One could figure out how a particular compiler
implements the v-table and access it using various casts and pointer
arithmetic. Not that I would suggest doing such a thing, but it is
possible nonetheless.

That is how Microsoft COM works. It "walks" the vtable. IIRC COM is an
external process so it has to get hold of the vtable just like any
programmer would have to. It helps a great deal to know how this
mechanism works.

To the OP, ignore people who tell you to "MYOB" or it's out of your
control so don't bother. People like that, regardless of their title
and/or education, are NOT engineers, software or otherwise.

Not all those who wander are lost. - J.R.R. Tolkien

[mlimber]

JustBoo wrote:

On 4 Jan 2006 09:24:59 -0500, "Meador Inge" <me*****@gmail. com> wrote:

Earl Purple wrote:

yp*********@ind iatimes.com wrote:
The v-table is beyond C++. You cannot access it directly, or get a
pointer to a v-table.

That is not 100% true. One could figure out how a particular compiler
implements the v-table and access it using various casts and pointer
arithmetic. Not that I would suggest doing such a thing, but it is
possible nonetheless.

That is how Microsoft COM works. It "walks" the vtable. IIRC COM is an
external process so it has to get hold of the vtable just like any
programmer would have to. It helps a great deal to know how this
mechanism works.

To the OP, ignore people who tell you to "MYOB" or it's out of your
control so don't bother. People like that, regardless of their title
and/or education, are NOT engineers, software or otherwise.

In my response, I wrote:

"The short answer: MYOB. It's implementation defined and not explicitly
accessible to your C++ program. See this FAQ (and the preceding one)
for some details:
http://www.parashift.c om/c++-faq-lite/virtual-functions.html# faq-20.4"

Since your comments are at least directed at me, please allow me to
respond.

The "MYOB" in my response was half joking. Of course, in some
relatively rare scenarios it might be necessary to figure out how a
compiler implements virtual functions and make use of that information
(n.b., compilers aren't required to use vtables at all and can
implement virtual functions with some other mechanism). However, as I
said, the language does not make that information *explicitly*
available to the user, and so the user *must* do some
platform-dependent pointer manipulation to figure out where the vtable
is and how to use it (assuming it exists at all!).

Therefore, since this newsgroup is exclusively concerned with
*standard* C++ language and libraries
(http://www.parashift.com/c++-faq-lit....html#faq-5.9), such
manipulations are, according to the FAQ, off-topic here and should be
taken to a more specific newsgroup that can treat them more adequately.

As far as implementing COM, Microsoft is free to make use of
non-standard information because it also controls the compiler. Another
company creating a COM-esque technology for some third-party
compiler(s) might pull the same sort of non-standard tricks, only to
find out on the next release of their compiler(s) that their tricks for
manipulating the vtable no longer work. That is because they used a
non-standard implementation detail from one version of their compiler
that the Standard allows the compiler vendor to change at will without
becoming non-conformant.

Finally, with regard to the aspersions and calumnies cast about in your
last sentence, I would humbly recommend that you dismount from your
elevated equine.

Cheers! --M

[Martin Bonner]

Dudle.June wrote:

every pointers has exactly same size

Not true in general.
a) On word addressed machines, void* and (unsigned) char * are longer
than other pointers because they contain the offset within the word.
b) In segmented architectures (like 16-bit Intel) it was quite common
for pointers to data and pointers to functions to be different lengths.
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[Francis Glassborow]

In article <11************ **********@o13g 2000cwo.googleg roups.com>,
Dudle.June <du*****@gmail. com> writes

every pointers has exactly same size
a pointer array usually has a type of void *
it can contain every kind of pointers
so ...

Actually your premise is false. Pointers to different types are not
required to be the same size. In addition a void* is not a correct
pointer type for a function pointer. When all is said and done compilers
implement late binding by creating internal tables of addresses, one per
class in a polymorphic hierarchy. The compler ensures that the same
function signature is applicable to the same slot in each of the tables.

--
Francis Glassborow ACCU
Author of 'You Can Do It!' see http://www.spellen.org/youcandoit
For project ideas and contributions: http://www.spellen.org/youcandoit/projects
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[william.wu1982]

May anyone tell me what exactly the pointer is ?
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[Mike Wahler]

<wi************ @gmail.com> wrote in message
news:11******** **************@ g14g2000cwa.goo glegroups.com.. .

May anyone tell me what exactly the pointer is ?

Which pointer? If you really meant to ask, what *a* pointer is,
it's an obect or value which represents a memory address.

-Mike

[Michiel.Salters]

wi************@ gmail.com wrote:

May anyone tell me what exactly the pointer is ?

No. We can't. It depends on a lot of factors, and we don't have a
crystal ball.
In fact, the next time you compile the profiler-optimizer may decide
not to
use a pointer at all.

HTH,
Michiel Salters
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[ypjofficial]

Hello All,

Thanks for all your help.
This discussion really help me understand the concept of the virtual
functions mechanisms.

Actually the reason behind asking this question is exactly what JustBoo
pointed out.I was loooking for interoperatibil y behind the c++ and C
when using the COM objects and how to access c++ vfptr from C..

I really appreciate all your help and guidance..
Hoping to get more help from this community in future..

Thanks and Regards,
Yogesh Joshi