virtual inhertence and its use in preventing inheritence

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C++

Hi,

If possible, can someone help and tell me what happens in internal when we use virtual inhertence to construct a sealed class?
I have the following two code snippet: The mere difference between the two is the virtual keyword. But if we don't use it, then , behaviour is not as expected.Please help me why it is so?

#include<iostream>

using namespace std;

class Base;

class final_lock
{
                friend class Base;
        private:
                final_lock() {}
                final_lock(const final_lock&) {}
};

class Base : public virtual final_lock { ///// Here virtual is necessary... why is it so? How does it make any difference internally?

public:
        Base() { }
        Base(char*) {}
        void fun(){ cout<<"I am in base"<<endl; }

};


class Der : public Base { };

int main()
{
        ///Der dd; // Cannot construct a Der obj... why it it so?
        ///dd.fun();
        Base m;
}

#include<iostream>

using namespace std;

class Base;

class final_lock
{
                friend class Base;
        private:
                final_lock() {}
                final_lock(const final_lock&) {}
};

class Base : public final_lock { //// Here virtual is not present.

public:
        Base() { }
        Base(char*) {}
        void fun(){ cout<<"I am in base"<<endl; }

};


class Der : public Base { };

int main()
{
        Der dd; /// Here we can construct a  Der obj!!!!!
        dd.fun();
        Base m;
}

I know a vptr is being constucted after use use virtual inheritence and the virtual base class is being constructed first. But how does it help in making sealed class? Please tell me internally what make it happen? Please let me know if I am not clear with my problem.