Question

I have this code :

A * a = new A;
a->fun();
delete a;

a = new B;
a->fun();
delete a;

What I need to do is to make it print :

A::fun() //being printed by A's fun()
B::fun() //being printed by B's fun()

without using the virtual keyword. Classes can be altered although the main code has to remain unchanged. How can this be done?

(Also, B is derived from A)

Answer 1

I hope this is for educational purposes.

If the destructor of a is not virtual this results in undefined behavior. So the destructor must be virtual.

If the destructor is virtual, you can use dynamic_cast. So, you can change A::foo to:

void A::fun()
{
   if ( dynamic_cast<B*>(this) )
       ((B*)this)->fun();
   else
       std::cout << "A::fun()";
}

Alternative with non-virtual destructor (undefined behavior if you call delete):

struct A
{
        bool isA;
        A(bool isA = true) : isA(isA){};
        void fun();
};
struct B : A
{
        B() : A(false){}
        void fun() {cout<<"B::fun()"<<endl;}
};

void A::fun()
{
        if (isA)
                std::cout << "A::fun()"<<endl;
        else
                ((B*)this)->fun();
}