A C++ program examples showing Polymorphism, please help.

well the interviewer didn't start out that way. She gave me codes in java, where a derived class overrides a function in the base class. She asked me if I invoke the overridden function of the derived class which is cast into its base class type, the base's or the derived's gets run. I said the derived's. She said good, then asked what if this is in C++. I said it'd be the same and she said NO, C++ works in the opposite way. I hereby seek enlightenment from you guys.

Code:

---

#include <iostream>
using namespace std;
class Base
{
public:
        int func_novirt();
        virtual int func_virt();
};

class Derived : public Base
{
public:
        int func_novirt();
        virtual int func_virt();
};

int Base::func_novirt (  )
{
        cout << "Base\n";
        return 0;
}

int Derived::func_novirt ( )
{
        cout << "derived\n";
        return 0;
}
int Base::func_virt (  )
{
        cout << "Base virtual\n";
        return 0;
}

int Derived::func_virt ( )
{
        cout << "derived virtual\n";
        return 0;
}


int main()
{
        Base b;
        Derived d;

        b.func_novirt ();
        d.func_novirt ();

        cout << "Virtuals\n";

        b.func_virt ();
        d. func_virt ();

        cout << "through base\n";
        Base *bp;
        bp = &b;
        bp->func_virt ();
        bp->func_novirt ();
        bp = &d;
        bp->func_virt ();
        bp->func_novirt ();

        return 0;
}

---

compile && run && study

Quote:

---

She asked me if I invoke the overridden function of the derived class which is cast into its base class type, the base's or the derived's gets run. I said the derived's. She said good, then asked what if this is in C++. I said it'd be the same and she said NO, C++ works in the opposite way. I hereby seek enlightenment from you guys.

---

The interviewer is correct. What she was asking about is type slicing, where an object of a derived class is cast to an object of its base class. Polymorphism in C++ works via pointers and references, so if she presented a case where a pointer to a derived class is cast to a pointer to the base class, then your answer would be correct.

This is an example that demonstrates type slicing and then polymorphic behaviour via a pointer:

Code:

---

#include <iostream>

class Base
{
public:
    virtual void foo() const
    {
        std::cout << "Base::foo()" << std::endl;
    }
};

class Derived : public Base
{
public:
    void foo() const
    {
        std::cout << "Derived::foo()" << std::endl;
    }
};

int main()
{
    Derived derived;

    static_cast<Base>(derived).foo(); // Cast from Derived to Base: type slicing occurs, so Base::foo() is called.

    Base* base_ptr = &derived; // Implicit cast from pointer to Derived to pointer to Base.
    base_ptr->foo();          // Polymorphism is used, so Derived::foo() is called.
}

---

I see. I actually tried writing similar codes to test it out but it didn't come out right. apparently there's one crucial difference. When coercing the derived into the base type I used explicit casting.

Thanks everyone again for the information. What makes really fun here is solving the problem I have in C/C++ programming together with everyone's help. I am really glad I come across to this site.


I have more questions to post here soon, so I hope everyone will be patient enough to teach me C and C++. God bless.


Respectfully Yours,

MarkSquall

P.S.
But how about Java? Is there a nice forum like this one about Java programming? Hope someone could share it too.