Adding virtual specifier in a derived class

Question

Consider following code:

struct virtualfoo 
{
    virtualfoo{};
    virtual ~virtualfoo{};

    virtual double doStuff() = 0
};


struct realbar :  virtualfoo   
{
     realbar{};
     virtual ~realbar{};

     virtual double doStuff();
};

Since I want to implement doStuff() for realbar, virtual isn't mandatory. But if I get this right, it won't hurt to have the virtual specifier next to realbar::doStuff(), does it? What side effects could I get with using/not using virtual?

Answer 1

The virtual keyword is not necessary in the derived class. However it makes code clearer. Also in C++11 override keyword is introduced which allows the source code to clearly specify that a member function is intended to override a base class method.

With keyword override the compiler will check the base class(es) to see if there is a virtual function with this exact signature. And if there is not, the compiler will throws an error.

Answer 2

It doesn't matter whether you explicitly declare realbar::doStuff as virtual, since it is implicitly virtual due to virtualfoo:doStuff being virtual. So no side effects; realbar::doStuff will be virtual anyway. Confer, for example, this online C++ draft standard:

10.3 Virtual functions

(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, parameter-type-list (8.3.5), cv-qualification, and ref- qualifier (or absence of same) as Base::vf is declared, then Derived::vf is also virtual (whether or not it is so declared) and it overrides Base::vf. ...