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]

"Kris Thielemans" <kr*************@imperial.ac.uk> wrote in message
news:6c**************************@posting.google.c om...

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.

#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.]

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" <kr*************@imperial.ac.uk> wrote in message
news:6c**************************@posting.google.c om...

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

[Rob Williscroft]

Kris Thielemans wrote in
news:6c**************************@posting.google.c om:

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

HTH

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

[tom_usenet]

On Wed, 08 Oct 2003 00:24:22 GMT, "Mike Wahler"
<mk******@mkwahler.net> wrote:

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

Did you mean to copy the object?

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

Tom

[Kris Thielemans]

"Mike Wahler" <mk******@mkwahler.net> wrote in message news:<WO****************@newsread3.news.pas.earthl ink.net>...

"Kris Thielemans" <kr*************@imperial.ac.uk> wrote in message
news:6c**************************@posting.google.c om...
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
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

[Ron Natalie]

"Kris Thielemans" <kr*************@imperial.ac.uk> wrote in message news:6c**************************@posting.google.c om...

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
}

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]

"Ron Natalie" <ro*@sensor.com> wrote in message news:<3f***********************@news.newshosting.c om>...

"Kris Thielemans" <kr*************@imperial.ac.uk> wrote in message news:6c**************************@posting.google.c om...

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

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.

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]

tom_usenet <to********@hotmail.com> wrote in message news:<bc********************************@4ax.com>. ..

On Wed, 08 Oct 2003 00:24:22 GMT, "Mike Wahler"
<mk******@mkwahler.net> wrote:

static_cast<A&>(b).func();
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

or just
b.A::func();

this works fine.

Kris

[tom_usenet]

On 9 Oct 2003 06:44:06 -0700, kr*************@imperial.ac.uk (Kris
Thielemans) wrote:

tom_usenet <to********@hotmail.com> wrote in message news:<bc********************************@4ax.com>. ..

On Wed, 08 Oct 2003 00:24:22 GMT, "Mike Wahler"
<mk******@mkwahler.net> wrote:

static_cast<A&>(b).func();

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().

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

Tom