Declare Whom Virtual??

**chottachatri** · 03-07-2008

Hello,
if a simple inheritance is there i am able to know as to which class should i declare as virtual!but when 6,7 classes are involved i get confused. Here is the code which i am posting

Code:

#include <iostream.h>
class A
{
	protected:
	int a;
};
class B:public A
{
	protected:
	int b;
};
class C:public B
{
	protected:
	int c;
};
class D:public C
{
	protected:
	int d;
};
class E:public D
{
	protected:
	int e;
};
class F:public B,public E
{
	protected:
	int f;
	public:
		void putdata()
		{
			cout<<a<<endl;
			cout<<b<<endl;
			cout<<c<<endl;
			cout<<d<<endl;
			cout<<e<<endl;
			cout<<f<<endl;

		}
};

int main(void)
{
F f;

return 0;
}

Here which classes should be declared as virtual ??i mean i know if i put the keyword virtual before B in class C declaration and in class E declaration i get no error. But how?????
Please state the reason and also explain me how to detect which class should be declared as virtual in complex derivation?

In the above derivation even A is inherited twice then why have we not declared A as virtual? please provide a detailed yet simple to understand explanation

**MacGyver** · 03-07-2008

This appears to be a good article on multiple inheritance and how virtual inheritance works:

http://www.phpcompiler.org/doc/virtualinheritance.html

**HyperShadow** · 03-07-2008

I wish I had never read that

.. but good stuff.

**chottachatri** · 03-08-2008

Nobody there to reply me????

**anon** · 03-08-2008

Originally Posted by chottachatri

Here which classes should be declared as virtual ??i mean i know if i put the keyword virtual before B in class C declaration and in class E declaration i get no error. But how?????
Please state the reason and also explain me how to detect which class should be declared as virtual in complex derivation?

In the above derivation even A is inherited twice then why have we not declared A as virtual? please provide a detailed yet simple to understand explanation

I don't get it. None of the classes has any methods (except F), so what would the virtual keyword do?

And then, what the hell is this "complicated" situation all about. It looks it is way more complicated than necessary. For example, F seems to introduce a "diamond pattern": it is multiply inherited from classes that share the same base class. Avoid such constructs until you are really an expert of C++.

**chottachatri** · 03-08-2008

You're right none of the classes has any methods but just for my reference i want to know..
Explain me where should be use virtual keyword in the above example and why??

**anon** · 03-08-2008

Uh oh, sorry, I didn't read the link and didn't actually know about virtual inheritance, so your question makes sense indeed.

However, if you don't want duplicate instantiation of bases, why would you inherit F from B and E. E is already a superclass of B, so why would you introduce the diamond pattern in the first place?

**CornedBee** · 03-08-2008

The rule is: if you want to use virtual inheritance to avoid multiple copies of base classes that are appear multiple times in the hierarchy (B in the example above), you must derive virtually exactly from the classes that appear multiple times:

Code:

class C : virtual public B

class F : public E, virtual public B

Don't forget the resulting initialization rules.

**matsp** · 03-08-2008

Also bear in mind that "protected" is essentially the same as public - you only need to create a new class that inherits the original class to freely access the protected member of the function. I'm not saying it's an entirely meaningless property of a class, but it's not very "protecting". In my opinion, member variables should be either public or private. Protected gives you a false sense of security.

--
Mats

**MacGyver** · 03-09-2008

Originally Posted by HyperShadow

I wish I had never read that

.. but good stuff.

I had that feeling when I read it.

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