Behavior of virtual function in C++

Question

I have a question, here are two classes below:

  class Base{
      public:
          virtual void toString();       // generic implementation
  }

  class Derive : public Base{
      public:
          ( virtual ) void toString();   // specific implementation
  }

The question is:

• If I wanna subclass of class Derive perform polymophism using a pointer of type Base, is keyword virtual in the bracket necessary?

• If the answer is no, what's the difference between member function toString of class Derive with and without virtual?

Answer 1

C++03 §10.3/2:

If a virtual member function vf is declared in a class Base and in a class Derived, derived directly or indirectly from Base, a member function vf with the same name and same parameter list as Base::vf is declared, then Derived::vf is also virtual (whether or not it is so declared) and it overrides Base::vf.