How to declare the nested classes correctly? [closed] - c++

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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 IDE 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.

Answer 1

This:
class LinkedBinaryTree {
protected:
struct Node;
is called a forward declaration (of Node). It tells the compiler there is a type with that name, but it is not a definition of a class. You cannot create an instance of a type, unless its definition is visible. A forward declaration is not a definition.
If you had written:
class LinkedBinaryTree {
protected:
struct Node { /* ... */ };
Then you would be able to create a Node.
The forward declaration of a nested class is generally useful when you have codependent types, or you prefer to order declarations in a specific way.
The forward declaration is useful to tell the compiler there is a type with that name, so it can understand your intent before it is used:
class LinkedBinaryTree {
protected:
struct Node;
void foo(Node*);
struct Node { /* ... */ };
In this case, a pointer does not require physical dependence, so the name is enough to satisfy the compiler when it sees the declaration of foo.
Also note that your forward declaration declares a Node and Position in the class' scope, so the compiler takes it to mean that void preorder(Node* v,PositionList& pl)const; uses the types local to the class; e.g. void LinkedBinaryTree::preorder(LinkedBinaryTree::Node* v, LinkedBinaryTree::PositionList& pl)const;, rather than the Position and Node declared in the global namespace in your code sample.

Answer 2

Nested declaration should normally happen inside the class, like this:
#include<iostream>
#include<list>
using namespace std;
class LinkedBinaryTree
{
protected:
typedef int Elem;
struct Node
{
Elem elt;
Node *par;
Node *left;
Node *right;
Node():elt(),par(NULL),left(NULL),right(NULL){}
};
public:
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;
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;}