private virtual Functions

Private virtual functions seem strange at first, and can be confusing, hence why many developers recommend that they are not used too often - if at all.

They are handy when functionality must be completely specific to a class, and therefore not callable from a derived class, even when a pointer to a base class is being used.

Consider these two classes:

class Parent
{
public:
  void Bar() {
    this->Whoami();
  }
 
private:
  virtual void Whoami() { 
    std::cout << "Parent::Foo" <<std::endl;
  }
};
 
class Child : public Parent
{
private:
   void Whoami() {      // virtual by default
    std::cout << "Child::Foo" <<std::endl;
  }
};

Now in this example, we create an instant of Child and assign to a Parent pointer:

int main()
{
  Parent* parent = new Child();
  parent->Bar();
  return 0;
}

Output:

Child::Foo