DaniWeb Forum Index > Software Development > C++

Virtual Inheritance Question

Please support our C++ advertiser: Intel Parallel Studio Home

Thread Solved

•

Join Date: Dec 2007

Posts: 449

Reputation: Agni is a jewel in the rough

Solved Threads: 70

Agni

Offline

Posting Pro in Training

Virtual Inheritance Question

Jul 9th, 2008

 Help with Code Tags 
 c++ Syntax (Toggle Plain Text) 
#include <iostream>
 
using namespace std;
 
class Lock
{
	friend class First;
	private:
		Lock()
		{
		}
};
 
class First : virtual public Lock
{
	public:
		void function()
		{
			cout << "function1" << endl;
		}
};
 
class Second: public First
{
	public:
		void function2()
		{
			cout << "function2" << endl;
		}
};
 
int main()
{
	First f;
	f.function();
 
	Second s;
	s.function2();
}
#include <iostream>

using namespace std;

class Lock
{
	friend class First;
	private:
		Lock()
		{
		}
};

class First : virtual public Lock
{
	public:
		void function()
		{
			cout << "function1" << endl;
		}
};

class Second: public First
{
	public:
		void function2()
		{
			cout << "function2" << endl;
		}
};

int main()
{
	First f;
	f.function();

	Second s;
	s.function2();
}

Error:
classVirtDer.cpp(39) : error C2248: 'Lock::Lock' : cannot access private member declared in class 'Lock'
classVirtDer.cpp(9) : see declaration of 'Lock::Lock'
classVirtDer.cpp(6) : see declaration of 'Lock'
This diagnostic occurred in the compiler generated function 'Second:

econd'

But when i remove the class Second, it works fine. Can someone explain?

thanks
-chandra

•

Join Date: Jun 2008

Posts: 973

Reputation: Alex Edwards is a jewel in the rough

Solved Threads: 107

Alex Edwards Alex Edwards is offline

Offline

Posting Shark

Re: Virtual Inheritance Question

Jul 9th, 2008

Friendship isn't inherited.

You declared a private constructor for the Lock class, in which it is only accessible either within that class or within a scope that has the right to access that Constructor.

Also because you've defined a constructor there is no default/dummy constructor provided.

Because First is a friend of Lock, First has the right to access Lock's private constructor.

Second does not inherit friendship of Lock despite the fact that Second derives from First.

In order to make the code work, you'll have to make Lock declare Second as a friend also--

 Help with Code Tags 
 c++ Syntax (Toggle Plain Text) 
#include <iostream>
 
using namespace std;
 
class Lock
{
	friend class First;
	friend class Second;
	private:
		Lock()
		{
		}
};
 
class First : virtual public Lock
{
	public:
		void function()
		{
			cout << "function1" << endl;
		}
};
 
class Second: public First
{
	public:
		void function2()
		{
			cout << "function2" << endl;
		}
};
 
int main()
{
	First f;
	f.function();
 
	Second s;
	s.function2();
	cin.get();
       return 0;
}
#include <iostream>

using namespace std;

class Lock
{
	friend class First;
	friend class Second;
	private:
		Lock()
		{
		}
};

class First : virtual public Lock
{
	public:
		void function()
		{
			cout << "function1" << endl;
		}
};

class Second: public First
{
	public:
		void function2()
		{
			cout << "function2" << endl;
		}
};

int main()
{
	First f;
	f.function();

	Second s;
	s.function2();
	cin.get();
       return 0;
}

Last edited by Alex Edwards; Jul 9th, 2008 at 2:57 am.

•

Join Date: Dec 2007

Posts: 449

Reputation: Agni is a jewel in the rough

Solved Threads: 70

Agni

Offline

Posting Pro in Training

Re: Virtual Inheritance Question

Jul 9th, 2008

i think its more because of the keyword 'virtual', just try and remove that and it compiles and excutes fine. Normally the class Second wil call the ctor of class First and then class First will call the ctor of Lock so i dont need to make 'Second' a friend of 'Lock'. but when i derive from the class 'Lock' virtually this doesnt happen.

thanks
-chandra

•

Join Date: Mar 2008

Posts: 41

Reputation: RenjithVR is an unknown quantity at this point

Solved Threads: 7

RenjithVR

Offline

Light Poster

Re: Virtual Inheritance Question

Jul 9th, 2008

http://www.computing.net/answers/pro...why/11546.html

•

Join Date: Jun 2008

Posts: 973

Reputation: Alex Edwards is a jewel in the rough

Solved Threads: 107

Alex Edwards Alex Edwards is offline

Offline

Posting Shark

Re: Virtual Inheritance Question

Jul 9th, 2008

I think I figured out the problem.

The link http://nxgenesis.blogspot.com/2007/1...-and-side.html states that Virtual Base classes are constructed before any Non-virtual Base class.

"virtual base classes and inheritance offer a set of rules which every c++ programmar should be aware of. Specifically:
virtual base classes are constructed before all non-virtual base classes.
virtual base class's constructor is directly invoked by the most-derived class's constructor."

When Second is instantiated, before the First constructor can run the Lock constructor runs before the First and because of that it's as if Second is trying to directly access Lock's private method since Second is the most-derived class and attempts to directly call the Lock constructor.

As stated previously, Friendship isn't inherited so even though First is a friend of Lock and Second derives from First, Second isn't a friend of Lock.

I'm fairly certain that this is the reason.

•

Join Date: Jul 2008

Posts: 338

Reputation: cikara21 is an unknown quantity at this point

Solved Threads: 64

cikara21

Offline

Posting Whiz

Re: Virtual Inheritance Question

Jul 9th, 2008

Change the private constructor into a public constructor..

.....???

Last edited by cikara21; Jul 9th, 2008 at 5:06 am.

$.:-cikara21-:.$

•

Join Date: Dec 2007

Posts: 449

Reputation: Agni is a jewel in the rough

Solved Threads: 70

Agni

Offline

Posting Pro in Training

Re: Virtual Inheritance Question

Jul 9th, 2008

thanks Edwards, that makes sense. Need to understand the virtual base classes more.

>Change the private constructor into a public constructor..

yes but then we wouldn't have faced the problem and wouldn't have come to know all this. it was just something i was trying to do.

thanks
-chandra