Final Class in c++

[Rajesh1978]

i got this program is in http://www.parashift.com/c++-faq-lit...l-classes.html

the code is

#include <iostream>
#include <memory>
using namespace std;

class Fred;

class FredBase {
    private:
          friend class Fred;
            FredBase() {cout<<"FredBase Cons"<<endl; }
};

class Fred : virtual private FredBase {
    public:
        Fred()
        {
            cout<<"Fred Cons"<<endl;
        }
};

class Leaf : public Fred
{
};

int main()
{
    Leaf f;
    return 0;
}

It works and do not allow to instantiate the Leaf object.

I have confusion about the virtual keyword in (class Fred : virtual private FredBase )

why it is required.
If i remove the virtual keyword it is allowing me to create the Leaf f object.

What is the importance of virtual here ?