Behavior of virtual function in C++

Question

Hi everyone:

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

That keyword there is strictly optional and makes no difference at all.

Answer 2

The virtual property is inherited from the base class and is assumed to be present even if you don't type it out.

Answer 3

The compiler already knows from the 'virtual' keyword in the base class that toString is a virtual method. No need to repeat it.

Answer 4

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.

Answer 5

It doesn't matter to the compiler whether or not you supply the virtual keyword on derived versions of the function.

However, it's a good idea to supply it anyway, so that anyone looking at your code will be able to tell it's a virtual function.

Answer 6

It's a matter of good style, and the user-programmer knows what's going on. In C++0x you can use [[override]] to make it more explicit and visible. You can use [[base_check]] to force the usage of [[override]].

If you don't want or can't do that, simply use the virtual keyword.

If you derive without virtual toString, and you cast an instance of Derive back to Base, calling toString() would actually call Base's toString(), since as far as it know's that's an instance of Base.