class Base
{
   public:
     virtual void fun() = 0;
    /* {
         cout<<"Base virtual fun"<<endl;
     }*/
};
class Derived : public Base
{
    public:
    void fun()
    {
       cout<<"Derived virtual fun"<<endl;
    }
};
int main()
{
   Derived d;
   d.fun();
   return 0;
}

Derived virtual fun