How to declare the nested classes correctly?

How to declare the nested classes correctly?

Following code is a implemention of Binary Tree, coming from a C++ data
structure text. I canot compile the code successfully, getting some error
messages. Mainly, the error lines comes from the last two of the code. How
to fix this problem? My compiler is CODE::BLOCK 12.11.
#include<iostream>
#include<list>
using namespace std;
typedef int Elem;
struct Node
{
Elem elt;
Node *par;
Node *left;
Node *right;
Node():elt(),par(NULL),left(NULL),right(NULL){}
};
class Position
{
private:
Node *v;
public:
Position(Node *_v=NULL):v(_v){}
Elem &operator*(){return v->elt;}
Position left()const{return Position(v->left);}
Position right()const{return Position(v->right);}
Position parent()const{return Position(v->par);}
bool isRoot()const{return v->par==NULL;}
bool isExternal()const{return v->left==NULL&&v->right==NULL;}
friend class LinkedBinaryTree;
};
typedef std::list<Position> PositionList;
class LinkedBinaryTree
{
protected:
struct Node; //This line is by me, the text merely tell you "insert Node
declaration here. . ." I don't know whether this line is correct or not.
public:
class Position; // Also by me, the text merely tell you "insert Position
declaration here. . ." I don't know wwhether this line is correct or not.
public:
LinkedBinaryTree();
int size()const;
bool empty()const;
Position root()const;
PositionList positions()const;
void addRoot();
void expandeExternal(const Position& p);
protected:
void preorder(Node* v,PositionList& pl)const;
private:
Node* _root;
int n;
};
LinkedBinaryTree::LinkedBinaryTree():_root(NULL),n(0){}
int LinkedBinaryTree::size()const{return n;}
bool LinkedBinaryTree::empty()const{return size()==0;}
LinkedBinaryTree::Position LinkedBinaryTree::root()const{Position(_root);}
//canot compile successfully, this error messages is :
C:\Users\user\Documents\aa\main.cpp|58|error: return type 'class
LinkedBinaryTree::Position' is incomplete
void LinkedBinaryTree::addRoot(){_root=new Node;n=1;} //canoot compile
successfully, this error message is
C:\Users\user\Documents\aa\main.cpp|59|error: invalid use of incomplete
type 'struct LinkedBinaryTree::Node'
There are many error messages, I select one of them to represent the error
messages.