Question

This is a different question than the other one I’ve posted, I have this piece of code:

class Base
{
public:
    Base()
    {

    };


    virtual void Method()
    {
        cout << "Base Method";
    }
};

class Derived : public Base
{
public:
    virtual void Method()
    {
        cout << "Override Method";
    }
};


class Derived2 : public Derived
{
public:

    Derived2()
    {

        cout << "Derived2 constructor";
    }
    void Method()
    {
        cout << "Override2 Method";
    }
};

int main()
{

    Base *myPointer = new Derived();

    dynamic_cast<Derived2*>(myPointer)->Derived2::Method();

    Sleep(700);

    delete myPointer;

    return 0;
}

If I write

dynamic_cast<Derived2*>(myPointer)->Method();

there’s a failure (dynamic_cast returns NULL and NULL->Method() provokes an exception) and this is what I was expecting, but if I write

dynamic_cast<Derived2*>(myPointer)->Derived2::Method();

the function succeeds without even calling the Derived2 constructor. Method isn’t even a static function, what is going on here?

Answer 1

You triggered undefined behavior by calling a member function on a NULL pointer. If you use dynamic_cast, you must either check the returned pointer for NULL before dereferencing it or 100% ensure you never cast to a type that is not the type of the object being cast or one of its parents.