Question

After upcasting a derived class’s pointer, a virtual method of the derived class is still called, which seems to me wrong, as slicing should have happened.
Could you please comment what’s wrong with this code?

class Base
{
public:
    virtual void Hello() { cout << "Hello Base" << endl; }
};

class Derived: public Base
{
public:
    void Hello() { cout << "Hello Derived" << endl; }
};

int main()
{
    Derived* der = new Derived;
    Base* base = dynamic_cast<Base*> (der);
    if (base) base->Hello();
}

output: Hello Derived

Answer 1

Slicing did not happen because you didn’t work with any values of Base, just pointers to it.

This would cause slicing:

Base base = *der;

But if you want to call a function and suppress dynamic dispatch, you can just do this:

base->Base::Hello();

The function to call is specified statically. This works with der too, of course, avoiding the middle-man.

---

Your dynamic_cast is not needed here. You can implicitly upcast, because this is trivially verifiable at compile-time. You can use static_cast to downcast, but it’s up to you to make sure this is actually correct; dynamic_cast is just a checked version of that.