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C++ question: dynamic_cast

Hi, all

I accidentally encountered the following code (not exactly of course that is very simplified version)

class V {
    virtual int getID() = 0;
};

class A : public V {
public:
    virtual int getID(){return 0;}
    virtual int getIDA(){return 10;}
};

class B  : public V {
public:
    virtual int getID(){return 1;}
    virtual int getIDB(){return 11;}
};

A *a = new A();
B *b = new B();


void *getA() {
    return a;
}

void *getB() {
    return b;
}

int main (int argc, char * const argv[]) {
    A *sa = static_cast<A *>(getB());
    A *aa = dynamic_cast<A *>(sa);
//here aa ISN'T NULL!!!!!

    return 0;
}


in that fragment 
aa isn't null
I'd expect NULL, of course
NOTE: gdb clearly shows that aa (or sa)  is an instance of the B class (with set print object on and set print vtbl on)
(gdb) p aa
$2 = (B *) 0x106940
(gdb) p *aa
$3 = (B) {
  <V> = {
    _vptr$V = 0x2078
  }, <No data fields>}
(gdb) 




from other hand
the following code behaves just fine

int main (int argc, char * const argv[]) {
    V *v = static_cast<V *>(getB());
    A *aa = dynamic_cast<A *>(v);
//here aa IS NULL which is good

    return 0;
}



thanks in advance for any help with clarification



Dmitry Markman


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