Confusion of friend with private inheritance

[CJIAJIA]

class Base{		
		int a;
	public:
		int b;
		Base(){a = 10; b = 11;}
		virtual ~Base(){};
		class Derived;
		friend Derived;
	};
class Derived: private Base{
	public:
		Derived(){cout<<b<<endl;}
		void p(Base b){cout<<b.a<<endl;};
		virtual ~Derived(){};
	};



int main(){
 Base b;
 Derived d;
 d.p(b);
 return 0;
}

I tried this code, and it can not compile in VC++. As my understanding, Derived is defined as Base's friend, so it can access Base's private member. Can anyone explain it? Thanks

[nuzzle]

Move

class Derived;

to before the Base class declaration.

[CJIAJIA]

Thanks a lot!. but what is the difference between putting the forward declaration inside and outside of the class?

[nuzzle]

Thanks a lot!. but what is the difference between putting the forward declaration inside and outside of the class?

Well it works if you do.

Just kidding. C++ allows you to declare so called inner classes and if you put a forward declaration within a class definition it's considered an inner class.

If you keep the forward declaration of Derived inside of Base and instead change Derived to,

class Base::Derived: private Base {

it should work too with the difference that Derived is now an inner class of Base.

[CJIAJIA]

you rock! Many thanks to your insightful explanation!

[Zachm]

I tried this code, and it can not compile in VC++. As my understanding, Derived is defined as Base's friend, so it can access Base's private member. Can anyone explain it? Thanks

This is a very strange way to achieve this.
If you have members in your base class that you need to access from Derived, declare them as protected.

Regards,
Zachm

[nuzzle]

you rock! Many thanks to your insightful explanation!

You're wellcome.

I've assumed that you're just testing things out so I've not commented on your code but as Zachm notes it's quite "unorthodox" in its present form.

[CJIAJIA]

Yes, Zachm and nuzzle, I am trying to using some tricky way to help me dig inside C++. It is not in the real programming.