使用dynamic_cast转换它们后删除指针：分配的内存发生了什么

Question

Let's say we have the following class hierarcy:

class B{     
    B() { cout<<"B\n"; }
    ~B(){ cout<<"~B\n"; }    
};

class D1 : virtual public B {
    D1() { cout<<"D1\n"; }
    ~D1(){ cout<<"~D1\n"; }
};

class D2 : virtual public B {
    D2() { cout<<"D2\n"; }
    ~D2(){ cout<<"~D2\n"; }
};

class MM : public D1, public D2 {
    MM() { cout<<"MM\n"; }
    ~MM(){ cout<<"~MM\n"; }
};

int main(){
    B *p = new MM();
    D1 *p2 = dynamic_cast<D1*>(p);
    D2 *p3 = dynamic_cast<D2*>(p);
    MM *p4 = dynamic_cast<MM*>(p);
    //delete p4;
    //delete p3;
    delete p2;
    delete p;
    return 0;
}

I'm not very used to dynamic_cast and pointers from base to derived classes so correct me if I'm wrong, but this basically creates a pointer of type B* to an object of type MM(). After this it creates another two pointers, and uses dynamic_cast to cast the B* pointer to the D*, D2* and MM*.

Now the thing that worries me is: the all four point to the same block of memory (right?). And when we delete p2, and then p, it works OK. The program crashes if we uncomment "delete p3" or delete "p4", and I've no idea why. I've alse observed that if we uncomment either of the previously mentioned delete statements and comment "delete p2" the code works fine.

Could someone care to explain? Thank you!

PS: Would any of you be so kind as to also explain what happens to the allocated memory once we create another pointer and use cast the previous one with dynamic_cast? Thanks!

Answer 1

Deleting class (without virtual destructor) via other type is undefined behaviour (UB).

Deleting pointer twice is also undefined behaviour (UB).

So your code may appear to work, but it is invalid too.