How can I set up my class so it won't be inherited from?

[Nikolay Kurtov]

http://www.parashift.com/c++-faq-lite/strange-inheritance.html#faq-23.11offers
a solution of the problem:

class Fred;

class FredBase {
private:
friend class Fred;
FredBase() { }
};

class Fred : private virtual FredBase {
public:
...
};


The only thing I want to know is why did he make FredBase a virtual
base. It works even if inheritance is non-virtual. Why virtual
inheritance is better??

 

[Victor Bazarov]

http://www.parashift.com/c++-faq-lite/strange-inheritance.html#faq-23.11offers
a solution of the problem:

class Fred;

class FredBase {
private:
friend class Fred;
FredBase() { }
};

class Fred : private virtual FredBase {
public:
...
};


The only thing I want to know is why did he make FredBase a virtual
base. It works even if inheritance is non-virtual. Why virtual
inheritance is better??

How does it work if the inheritance non-virtual? What error does
your compiler report here:

class Fred;

class FredBase {
private:
friend class Fred;
FredBase() { }
};

class Fred : private /*virtual*/ FredBase {
};

class Foo : public Fred {};

class Bar : private Fred {};

int main() {
Foo foo;
Bar bar;
}

I've derived TWO classes from Fred and my compiler had no problem
letting me.

V

 

[Nikolay Kurtov]

I've derived TWO classes from Fred and my compiler had no problem

letting me.

V
--
Please remove capital 'A's when replying by e-mail
I do not respond to top-posted replies, please don't ask- Hide quoted text -

- Show quoted text -

Sorry, it really produces an error.
Now I really confused and cannot understand why it is possible to
derive.
Isn't it true, that a usual inheritance runs first constructor of the
base class and then of the derived one?

 

[Nikolay Kurtov]

Sorry, it really produces an error.
I meant without "virtual" it IS possible to derive from Fred.

 

[Victor Bazarov]

Sorry, it really produces an error.

Who "it"? What error?

Now I really confused and cannot understand why it is possible to
derive.

It's possible to derive from _my_ Fred (that doesn't have 'FredBase'
as virtual base class) because NOTHING prevents you from that.

Isn't it true, that a usual inheritance runs first constructor of the
base class and then of the derived one?

The virtual base class subobject is supposed to be _initialised_ in
the most derived class' constructor. If you derive from [the original]
'Fred', say a class 'Foo', then 'Foo::Foo' is responsible for
initialising the 'FredBase' (even if it happens implicitly), and since
'FredBase's constructor is private, it's not accessible to 'Foo' (or
any other class derived from 'Fred' for that matter). That's why you
cannot derive from 'Fred' given in the FAQ. The inability to initialise
the virtual base class in any derived from 'Fred' class makes 'Fred'
non-derivable.

V

 

[Nikolay Kurtov]

OK, thanks a lot.

 

[vlad]

OK, thanks a lot.

Nikolay,

I have had the same issue too and created a generic solution for this problem. You can find free examples and source code here - http://www.jetsnail.com/tech-blog/38-cpp/52-noninheritable. Do not hesitate to ask me should you have any questions. Hope it helps! :beerglas: