C / C++ / MFC

13-Jan-10 21:49

Hi,

Have a doubt, will virtual destructor work only when u create objects thru heap ??? Bcoz the following code works either way with or without virtual destructor .

class A
{
int *p;
public :
A() {p = new int;
*p = 10;}
~A ()
{
delete p;
p = NULL;
printf("this is A dest \n");
}

virtual int getVal()
{
if(p)
return *p;
return 0;
}
};

class B : public A
{
int *p1;
public :
B() {
p1 = new int;
*p1 = 10;
printf("this is B dest \n");
}
~B ()
{
delete p1;
p1 = NULL;
}

int getVal()
{
if(p1)
return *p1;
return 0;
}
};

int main(int argc, char* argv[])
{
B b;
A *ap = &b;
ap->getVal();

//But virtual destructor works when u do thru heap
//B *pb = new B;
//A *ap = new B;
//delete pb;
//delete ap;
return 0;
}

Here when objects goes out of scope , both destructors getting called irrespecive of virtual. So virtual destructor works only with heap ?

CPallini13-Jan-10 22:31

CPallini

13-Jan-10 22:31

There's nothing strange in that: when b goes out of scope, its destructor is called (as well its base class one). b type is know at compile time.
Smile | :)

Graham Breach13-Jan-10 22:37

Graham Breach

13-Jan-10 22:37

You've got your printf("this is B dest \n"); in the constructor for the B class, instead of the destructor.

Stuart Dootson13-Jan-10 23:27

13-Jan-10 23:27

You *need* a virtual destructor when you could possibly be destructing an object of type B through a pointer or reference of type A.

But virtual destructors can *always* be used in place of non-virtual destructors, whether objects are allocated on heap or stack.

Java, Basic, who cares - it's all a bunch of tree-hugging hippy cr*p

MVP for 2010 - who'd'a thunk it!

davarun15-Jan-10 2:36

15-Jan-10 2:36

hey, so virtual destructor wont work if u the objects created at stack ? Bcoz in my example , A::destructor is called bcoz B is going out of scope. If my statement wrong can u pl give me sample with objects created in stack to test Virtual destructor.

thanks

Stuart Dootson15-Jan-10 2:58

15-Jan-10 2:58

Remember that B's destructor will call A's destructor...once it's finished any of your code in B's destructor. That is why A's destructor is being called.

Oh - and also, you won't see if B's destructor is called, because your print statement is in B's CONSTRUCTOR, not B's DESTRUCTOR!

Java, Basic, who cares - it's all a bunch of tree-hugging hippy cr*p

MVP for 2010 - who'd'a thunk it!

davarun15-Jan-10 3:23

15-Jan-10 3:23

hi thanks,

(Ya i was put inside constructor by mistake - Smile | :)

)

I understand that when B goes out of scope, A's destructor also called . But is there a way to see that virtual destructor is called for objects created at stack .

thanks

Stuart Dootson15-Jan-10 3:38

15-Jan-10 3:38

I really don't understand what you mean.

Java, Basic, who cares - it's all a bunch of tree-hugging hippy cr*p

MVP for 2010 - who'd'a thunk it!

davarun15-Jan-10 4:34

15-Jan-10 4:34

I mean in my example,

int main()
{

B b;
A *ap = &b;
ap->getVal();//when B object goes out of scope, B's desc is called first then A's (here its not thru virtual destr)

//here i see virtual destructor is called ,(ie B'destc first then A's destr )
A *pb = new B;
delete pb;
}

So here i m trying to see B's destr called thru virtual destructor concept.

Stuart Dootson15-Jan-10 4:44