calling virtual function that is hidden by inheritance

[Kris Thielemans]

Hi

I have a rather outlandish problem where I want to call a virtual
function which is (sort of) hidden by a derived class. For instance

class A { virtual void func(); };
class B: public A { virtual void func(); }

void some_routine()
{ B b;
b.func(); // will call B::func
??? // how to call A::func() for the b object
}

I know how to do this within the definition of a member. For instance
void B::func()
{
A::func();
}

but I need it outside the class.

In case you wonder why I need it... I am writing a test program. In my
case A::func() and B::func() should give the same results, but
B::func() does it faster. My test program wants to check if the
results are indeed the same.

Thanks for any help!

Kris

[Mike Wahler]

Re: calling virtual function that is hidden by inheritance


"Kris Thielemans" <kris.thieleman s@imperial.ac.u k> wrote in message
news:6c1545b0.0 310071556.50a3e faf@posting.goo gle.com...[color=blue]
> Hi
>
> I have a rather outlandish problem where I want to call a virtual
> function which is (sort of) hidden by a derived class. For instance
>
> class A { virtual void func(); };
> class B: public A { virtual void func(); }
>
> void some_routine()
> { B b;
> b.func(); // will call B::func
> ??? // how to call A::func() for the b object
> }
>
> I know how to do this within the definition of a member. For instance
> void B::func()
> {
> A::func();
> }
>
> but I need it outside the class.
>
> In case you wonder why I need it... I am writing a test program. In my
> case A::func() and B::func() should give the same results, but
> B::func() does it faster. My test program wants to check if the
> results are indeed the same.[/color]

#include <iostream>

class A
{
public:
virtual void func() { std::cout << "A\n"; }
};

class B: public A
{
public:
virtual void func() { std::cout << "B\n"; }
};

int main()
{
B b;
b.func(); /* prints "B" */
static_cast<A>( b).func(); /* prints "A" */
return 0;
}

-Mike

[Ali R.]

Re: calling virtual function that is hidden by inheritance

Hi Kris,

Mike's way is one way. Fortunately this is more than one way to skin the
cat. I like this one better :
what you do is use the name of the parent class just like a member variable
followed by ::

class Parent
{
public:
virtual void func() { std::cout << "A\n"; }
};

class Child : public Parent
{
public:
virtual void func() { std::cout << "B\n"; }
};

int main()
{
Child b;

b.func(); /* prints "B" */
b.Parent::func( ); /* prints "A" */

return 0;
}


Ali R.

"Kris Thielemans" <kris.thieleman s@imperial.ac.u k> wrote in message
news:6c1545b0.0 310071556.50a3e faf@posting.goo gle.com...[color=blue]
> Hi
>
> I have a rather outlandish problem where I want to call a virtual
> function which is (sort of) hidden by a derived class. For instance
>
> class A { virtual void func(); };
> class B: public A { virtual void func(); }
>
> void some_routine()
> { B b;
> b.func(); // will call B::func
> ??? // how to call A::func() for the b object
> }
>
> I know how to do this within the definition of a member. For instance
> void B::func()
> {
> A::func();
> }
>
> but I need it outside the class.
>
> In case you wonder why I need it... I am writing a test program. In my
> case A::func() and B::func() should give the same results, but
> B::func() does it faster. My test program wants to check if the
> results are indeed the same.
>
> Thanks for any help!
>
> Kris[/color]

[Rob Williscroft]

Re: calling virtual function that is hidden by inheritance

Kris Thielemans wrote in
news:6c1545b0.0 310071556.50a3e faf@posting.goo gle.com:
[color=blue]
>
> void some_routine()
> { B b;
> b.func(); // will call B::func
> ??? // how to call A::func() for the b object[/color]

b.A::func();
[color=blue]
> }
>[/color]

HTH

Rob.
--

http://www.victim-prime.dsl.pipex.com/

[tom_usenet]

Re: calling virtual function that is hidden by inheritance

On Wed, 08 Oct 2003 00:24:22 GMT, "Mike Wahler"
<mkwahler@mkwah ler.net> wrote:
[color=blue]
>int main()
>{
> B b;
> b.func(); /* prints "B" */
> static_cast<A>( b).func(); /* prints "A" */[/color]

Did you mean to copy the object?

static_cast<A&> (b).func();
or just
b.A::func();

Tom

[Kris Thielemans]

Re: calling virtual function that is hidden by inheritance

"Mike Wahler" <mkwahler@mkwah ler.net> wrote in message news:<WOIgb.175 7$av5.774@newsr ead3.news.pas.e arthlink.net>.. .[color=blue]
> "Kris Thielemans" <kris.thieleman s@imperial.ac.u k> wrote in message
> news:6c1545b0.0 310071556.50a3e faf@posting.goo gle.com...[/color]

Thanks Mike

however it seems that your solution doesn't work for me (using gcc
3.2). The reason being that my base class A has other pure (and
unimplemented) virtual members. When I do the static_cast<A> trick I
get an error "cannot allocate an object of type A because the
following virutal functions are abstract ..."

In contrast, the suggestion by Ali and Rob (to use b.A::func()) works
fine in that case as well.

Thanks all 3 of you.

Kris
[color=blue]
> class A
> {
> public:
> virtual void func() { std::cout << "A\n"; }
> };
>
> class B: public A
> {
> public:
> virtual void func() { std::cout << "B\n"; }
> };
>
> int main()
> {
> B b;
> b.func(); /* prints "B" */
> static_cast<A>( b).func(); /* prints "A" */
> return 0;
> }
>
> -Mike[/color]

[Ron Natalie]

Re: calling virtual function that is hidden by inheritance


"Kris Thielemans" <kris.thieleman s@imperial.ac.u k> wrote in message news:6c1545b0.0 310071556.50a3e faf@posting.goo gle.com...
[color=blue]
> class A { virtual void func(); };
> class B: public A { virtual void func(); }
>
> void some_routine()
> { B b;
> b.func(); // will call B::func
> ??? // how to call A::func() for the b object
> }
>[/color]

You can't do this. That's the whole point of access control.
You can't access A's private parts. It's got nothing whatsoever
to do with virtual or hiding.

[Kris Thielemans]

Re: calling virtual function that is hidden by inheritance

"Ron Natalie" <ron@sensor.com > wrote in message news:<3f84281b$ 0$37005$9a6e19e a@news.newshost ing.com>...[color=blue]
> "Kris Thielemans" <kris.thieleman s@imperial.ac.u k> wrote in message news:6c1545b0.0 310071556.50a3e faf@posting.goo gle.com...
>[color=green]
> > class A { virtual void func(); };
> > class B: public A { virtual void func(); }[/color]
>
> You can't do this. That's the whole point of access control.
> You can't access A's private parts. It's got nothing whatsoever
> to do with virtual or hiding.[/color]

oops. sorry. forgot the public there! You're right of course. but the
other posters gave me the answer I wanted anyway. lucky me.

thanks

kris

[Kris Thielemans]

Re: calling virtual function that is hidden by inheritance

tom_usenet <tom_usenet@hot mail.com> wrote in message news:<bck7ov8mh 8dskvulkpp5lqko i7ogi6ki4l@4ax. com>...[color=blue]
> On Wed, 08 Oct 2003 00:24:22 GMT, "Mike Wahler"
> <mkwahler@mkwah ler.net> wrote:
>
> static_cast<A&> (b).func();[/color]

right, that would solve my pure virtual problem I mentioned.

However, this static_cast trick actually does not work. It will still
call B::func(). I checked this with gcc, but I find the easiest
explanation as follows:

In my opinion, the above is functionally identical to

A& a_ref = b; /* feel free to insert a static_cast here, but you
don't have to*/
a_ref.func();

Now, I hope you'll agree that the last statement definitely calls
B::func(). Otherwise there would be no point in having virtual
functions. Indeed suppose you have a function that works on A&
objects, which version of func() do you want it to call normally?

void some_other_func (A& a)
{
a.func();
}
some_other_func (b); // will call B::func



[color=blue]
> or just
> b.A::func();
>[/color]

this works fine.

Kris

[tom_usenet]

Re: calling virtual function that is hidden by inheritance

On 9 Oct 2003 06:44:06 -0700, kris.thielemans @imperial.ac.uk (Kris
Thielemans) wrote:
[color=blue]
>tom_usenet <tom_usenet@hot mail.com> wrote in message news:<bck7ov8mh 8dskvulkpp5lqko i7ogi6ki4l@4ax. com>...[color=green]
>> On Wed, 08 Oct 2003 00:24:22 GMT, "Mike Wahler"
>> <mkwahler@mkwah ler.net> wrote:
>>
>> static_cast<A&> (b).func();[/color]
>
>right, that would solve my pure virtual problem I mentioned.
>
>However, this static_cast trick actually does not work. It will still
>call B::func().[/color]

Whoops, of course, since func is virtual (I hadn't spotted that).

Tom