Make implementation of virtuals also virtual?

When implementing a pure virtual function in C++, is there a best-practices guideline开发者_如何转开发 that says the implementation should also be made virtual? What is the rationale?

class Interface
{
  public:
    virtual void foobar() = 0;
};

class Concrete
    : public Interface
{
  public:
    virtual void foobar();
};

It does not matter.

void foobar() in Concrete is virtual regardless whether you declare it as such and it overrides the void foobar() in Interface.

Although it doesn't matter if the virtual keyword is present in a derived class or not, I've found it to be an indispensable time-saving self-documenting practice to always include it, so that anyone working with your code a two years from now immediately can see that there is more to the class than what immediately meets the eye.