C++ - Stop people deriving from your class

One of my friends was facing a problem while trying to write a class B so that no one would be able to inherit from it. He used code as below.

---

class B; // forward declaration



class Prot // this class is used to stop the inheritance

{

public:

    friend class B; // allow only B access to ctors

private:

    Prot(){}; // make all ctors private

    Prot(const Prot &p){};   

};



// class that cannot be inherited from

class B: public Prot // inherit from prot. Since B is friend of Prot it'll work

{

public:

    B();

    int a;

};



class D : public B

{

public:

    int d;

};



D d; // this should fail

---

However D was still able to inherit from B. What he missed was using virtual inheritance for inheriting from Prot as follows

class B: public virtual Prot // inherit from prot

{

 // ...

}

this lead to the bug. Without virtual inheritance the call does not go via the vtable and D is able to access Prot's ctor as it is merged with that of B.