C++ friend, override nested class's function

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::")

That fact that `Parent` is a subclass of `Child` does not mean it automatically generates a subclass of `Nested`. Instead, it inherits the entire nested class. And btw., why are you using both `friend`-ship and inheritance? — Fred Foo, Jul 12 '12 at 09:35
I use friendship because "Nested" is private and I use inheritance because I want don't want to rewrite the whole "Parent" just alter that one function. — user1518399, Jul 12 '12 at 09:45

score 10 · Accepted Answer · answered Jul 12 '12 at 09:33

10

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.

answered Jul 12 '12 at 09:33

Luchian Grigore

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Does this mean that an instance of `Child` will thus have two nested classes, `Nested` and `ChildNested` – johnbakers May 25 '13 at 05:37
And incidentally, why is it necessary to make `Child` a `friend` if `Child` derives? I would think that a public inheritance would allow Child to access Nested fine – johnbakers May 25 '13 at 05:38

score 2 · Answer 2 · answered Jul 12 '12 at 09:34

Basically you need to derive another class in the Child as given below and implement your virtual function there.

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

class Child : Parent 
{

    //derive a new class in the child to override the virtual function.
    class ChildNested : public Parent::Nested
    {
       void nestedfun() {std::cout << "two";}
    };
};

score 1 · Answer 3 · answered Jul 12 '12 at 09:33

You can overwrite virtual function of the base class in the derived class. Inner class is a separate class. You need to create a class that will derive from your inner class. Something like that:

class Child : Parent::Nested {
  void nestedfun() {std::cout << "two";}
}

C++ friend, override nested class's function

3 Answers3

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