关于动态铸件和（钻石）inheritance

Question

I have the following code snipped:

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


class B{
int i;
public:
    B(){i = 1;}

virtual int get_i(){return i;}

};

class D: virtual public B{
    int j;
public:
    D(){j = 2;}
    int get_i(){return B::get_i() + j;}
};

class D2: virtual public B{
    int j2;
public:
    int get_i(){return B::get_i() + j2;}
};


class MM: public D, public  D2{
    int x;
public:
    MM(){
        x = D::get_i() + D2::get_i();
    }
    int get_i(){return x;}
};

int main(){
    B* o = new MM();
    cout << o->get_i() << "\n";
    MM* p = dynamic_cast<MM*>(o);
    if(p) cout << p->get_i() << "\n";
    D* p2 = dynamic_cast<D*>(o); /// Why does this dynamic_cast not return NULL?
    if(p2)
        cout << p2->get_i() << "\n";
}

The output will be:

My question is: Why does the dynamic_cast on the marked line success? So, if I have an hierarchy like:

class Grandpa{...};
class Dad : public Grandpa{...};
class Son: public Dad{...};

Of course that

Grandpa* grandpa = new Son();

Is ok.

Will the following dynamic_cast :

Dad* dad = dynamic_cast<Dad*>(grandpa);

always succeed?

It's somehow logical, as long as a Dad pointer always can point to a Son object, but I have never seen this before, and I want to assure that I can take that as a general rule.

Answer 1

Why does the dynamic_cast on the marked line success?

Because the dynamic type of the object has a D base.

Will the following dynamic_cast:

 Dad* dad = dynamic_cast<Dad*>(grandpa);

always succeed?

Given the premise of Grandpa* grandpa = new Son(); , yes it will succeed.

But it won't succeed in other cases such as for example:

Grandpa gp;
Grandpa* grandpa = &gp;
Dad* dad = dynamic_cast<Dad*>(grandpa); // is null

Answer 2

Yes, as long as the object pointed to by the superclass-type pointer really is an object of the type you are trying to cast to, dynamic_cast will succeed. If it isn't an object of that type, otoh, dynamic_cast will return a null pointer.