C++ Access private member of nested class

Question

The title might be a bit misleading. I have the following problem: I have a tree consisting of leaves and internal nodes. The user should be able to store any information in

Answer 1

You can declare class Tree a friend to UserElement<>, which would allow Tree to access all members of UserElement<>.

Answer 2

You may do

class Outer {
   private: // maybe protected:
   class Inner {
      public:
      ....
   };
};

or

class Outer {
   public:
   class Inner {
      friend class Outer;
      private:
      ....
   };
};

Answer 3

Technically, that's a nested class (declared within another class), not a subclass (which inherits from its superclass).

You can allow the Tree class to access its privates by making it a friend:

class UserElement {
    friend class Tree;
    // ...
};

or, for better encapsulation, you could restrict access only to the member function(s) that need it, although it gets a bit messy due to the need to declare things in the right order:

class Tree {
public:
    // Declare this so we can declare the function
    template <typename T> class UserElement;

    // Declare this before defining `UserElement` so we can use it
    // in the friend declaration
    template <typename T>
    void doSomethingWithTheTree(list<UserElement<T>> elements) {
        elements.front().leaf;
    }

    template <typename T>
    class UserElement {
        // Finally, we can declare it a friend.
        friend void Tree::doSomethingWithTheTree<T>(list<UserElement<T>>);
        // ...
    };
};