Pure Virtual Function - Virtual Method Table

u_peerless · #1 06-Jun-2008, 05:15

Hi C++ Coders,
Can you please help me regarding one thing.
For pure virtual function, what are the entries created in the virtual method table.
Say I have a program:

CPP / C++ / C Code:

class base
{
   public:
            virtual void display() = 0;
         ...........
};

class derived1:public base
{
  public:
           void display()
            {
               cout<<"Derived1:";
            }
};

class derived2:public derived
{
  public:
           void display()
            {
               cout<<"Derived2:";
            }
};

So here when derived class functions will be called through base class pointer, then how with the help of virtual method table the proper function is calling?
And what are the entries in the 3 vtable for these 3 classes?

And one more query:
To see vtable core dump, is there any compiler flag (g++) in Solaris / Red Hat Linux to be used?

Thanks in advance.

With regards,
u_peerless

Peter_APIIT · #2 06-Jun-2008, 06:07

Virtual Table is available for only pure virtual function.

Quote:

then how with the help of virtual method table the proper function is calling?

Basically, virtual table is just sort of array of function pointer.

RTTI helps to investigate the object actual content type.

After investigation, they will just called the correct class for the class.

I hope this help.

Please correct me if i wrong.

u_peerless · #3 06-Jun-2008, 07:40

Virtual Table is created even if it is not pure virtual function.
I got my answer, I posted this question in another forum. I got reply from them.
cboard.cprogramming.com
Thanks for your reply.

ocicat · #4 06-Jun-2008, 08:46

Quote:

Originally Posted by Peter_APIIT

Virtual Table is available for only pure virtual function.

No. The consequence of declaring pure virtual functions is the compiler will enforce that instances of this type cannot be created.

Peter_APIIT · #5 07-Jun-2008, 17:52

That's means We cannot create object from abstract class or classes with pure virtual function.

Please post the link here and let other benefits.

ocicat · #6 07-Jun-2008, 17:59

Quote:

Originally Posted by Peter_APIIT

That's means We cannot create object from abstract class or classes with pure virtual function.

Correct. Unless all virtual member functions can be resolved (meaning that valid entries for each virtual member function specified in the vtable...), objects of that type cannot be created. This is why these classes are called abstract.

However, note that pointers to abstract classes can be created.

Quote:

Please post the link here and let other benefits.

All C++ textbooks should mention this fact. If they don't, they aren't worth the paper they are printed upon.

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Pure Virtual Function - Virtual Method Table

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