How compiler will determine the parent in virtual derivation

Poolan · #1 11-Feb-2005, 05:34

Hi

I have a base class A. I have derived two classes B & C from A virtually. After that I have derived Class D from B and C. Class have a function fun().
I have created One object of D say obj and when I call obj.fun() it will call fun() of A.
Till now everything is fine.

But what is wondering me is how compiler knows weather fun() is coming to D through B or C as both have one copy of fun() and D also will have only one.

Poolan

QED · #2 12-Feb-2005, 09:13

I would like to answer your question, but I don't completely understand your example. Could provide more detail, including the context in which you use obj of type D and a code example?

Matthew

Poolan · #3 14-Feb-2005, 06:32

CPP / C++ / C Code:

class A { public: fun() { } };
class B : virtual public A { };
class C : virtual public A { };
class D : public B, public C { }; 

D obj;
obj.fun();

We can call like this. right?

If the virtual keyword is not there while deriving B and C from A we will have two copies of fun() in D. right?

But the virtual keyword will avoi that.

I would like to know how the compiler knows that the fun() is coming to D through B or C.

QED · #4 14-Feb-2005, 06:59

e base class is instantiated only once in any derived class. So using virtual when inheriting from A in the intermediate classes B and C is enough to avoid instantiating multiple copies of base class A, even during the instantiation of a D object.

Placing the keyword virtual in the declaration for D, say

CPP / C++ / C Code:

class D : virtual public B, virtual public C { };

will ensure that instances of B and C are created only once in the case that we inherit from class D, but does not directly affect how many copies of A we will have.

Hence, the following code should not compile.

CPP / C++ / C Code:

class A {
public:
  void fun() { }
};

class B : public A { };

class C : public A { };

class D : virtual public B, virtual public C { };

int main() {
  D obj;
  obj.fun();
  return 0;
}

Am I explaining this well?

Matthew

Poolan · #5 14-Feb-2005, 07:15

Hi,

Thank you for your detailed reply.

Will you clarify one more thing. I read in one book that there is one extra pointer along with the class in case of virtual derivation (Not the VPTR, which is for virtual funs, etc). Is the single time instanciation of the class is handled through this pointer.

From where can I get the details about this. Can you please ref one good book for me. I am very eager to know the internals of C++.

machinated · #6 14-Feb-2005, 08:46

you referring to the this pointer?

Poolan · #7 15-Feb-2005, 05:57

No, There is another one

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