Question

I want to override a virtual function in a nested class, from a friend (+child) class. How is this possible? Here is what I tried:

class Parent {
  friend class Child;
  class Nested {
    public: virtual void nestedfunc() {std::cout << "one";}
  }
};
class Child : Parent {
  void Child::Nested::nestedfun() {std::cout << "two";}
}

But I get:

error: cannot define member function 'Parent::Nested::nestedfunc' within 'Child'

(Same error without “Child::”)

Answer 1

As overrides go, you need to extend the class and then override it:

class Parent {
  friend class Child;
  class Nested {
    public: virtual void nestedfunc() {std::cout << "one";}
  }
};
class Child : Parent {
  class ChildNested : Parent::Nested
  {
    public: virtual void nestedfunc() {std::cout << "two";}
  }
}

Inner classes aren’t that special, they don’t break the rules of inheritance or provide any special magic that would let you override a method but not derive from the class that declares it.