C++ Forward Declaration of Objects

Question

i'm trying to figure out how to forward declare objects where new objects are storing other new objects for example:

A Queue, holding Nodes, these Nodes hold Data items which hold 2 Lists or preferably 2 Queues.

Here is my Data.h

#ifndef DATA_H_
#define DATA_H_

class List;
class Data {
public:
    Data();
    Data(int aID);
    virtual ~Data();
    virtual int getID();
    virtual void setID(int aID);
private:
    int ID;
    List *listOne;
    List *listTwo;
};

#endif /* DATA_H_ */

And my Data.c

#include "Data.h"
#include "List.h"

Data::Data() {
    listOne = new List();
    listTwo = new List();
    Data::ID = 0;
}

Data::Data(int aID) {
    Data::listOne = new List();
    Data::listTwo = new List();
    Data::ID = aID;
}

Data::~Data() {
    delete(listOne);
    delete(listTwo);
}

int Data::getID() {
    return Data::ID;
}

void Data::setID(int aID) {
    Data::ID = ID;
}

The error message:

..\Data.cpp: In constructor 'Data::Data()':
..\Data.cpp:12:23: error: invalid use of incomplete type 'class List'
  listOne = new List();
                       ^
In file included from ..\Data.cpp:8:0:
..\Data.h:11:7: note: forward declaration of 'class List'
 class List;
       ^
..\Data.cpp:13:22: error: invalid use of incomplete type 'class List'
  listTwo = new List();
                      ^
In file included from ..\Data.cpp:8:0:
..\Data.h:11:7: note: forward declaration of 'class List'
 class List;
       ^
..\Data.cpp: In constructor 'Data::Data(int)':
..\Data.cpp:18:29: error: invalid use of incomplete type 'class List'
  Data::listOne = new List();
                             ^
In file included from ..\Data.cpp:8:0:
..\Data.h:11:7: note: forward declaration of 'class List'
 class List;
       ^
..\Data.cpp:19:28: error: invalid use of incomplete type 'class List'
  Data::listTwo = new List();
                            ^
In file included from ..\Data.cpp:8:0:
..\Data.h:11:7: note: forward declaration of 'class List'
 class List;
       ^
..\Data.cpp: In destructor 'virtual Data::~Data()':
..\Data.cpp:24:18: warning: possible problem detected in invocation of delete operator: [-Wdelete-incomplete]
  delete(listOne);
                  ^
..\Data.cpp:24:18: warning: invalid use of incomplete type 'class List'
In file included from ..\Data.cpp:8:0:
..\Data.h:11:7: note: forward declaration of 'class List'
 class List;
       ^
..\Data.cpp:24:18: note: neither the destructor nor the class-specific operator delete will be called, even if they are declared when the class is defined
  delete(listOne);
                  ^
..\Data.cpp:25:17: warning: possible problem detected in invocation of delete operator: [-Wdelete-incomplete]
  delete(listTwo);
                 ^
..\Data.cpp:25:17: warning: invalid use of incomplete type 'class List'
In file included from ..\Data.cpp:8:0:
..\Data.h:11:7: note: forward declaration of 'class List'
 class List;
       ^
..\Data.cpp:25:17: note: neither the destructor nor the class-specific operator delete will be called, even if they are declared when the class is defined
  delete(listTwo);

Now if I remove the forward delclaration in Data.h and have Data.h include List.h I will get: and result in a circular inheritance cycle

..\Data.h:25:2: error: 'List' does not name a type
  List *cpubursts;
  ^
..\Data.h:26:2: error: 'List' does not name a type
  List *iobursts;

How will I go about fixing this sort of setup?

Edit: List.h

#ifndef LIST_H_
#define LIST_H_

#include "Node.h"

namespace std {

class List {
private:
    Node *top;
public:
    List();
    virtual ~List();
    virtual Node* getTop();
    virtual void add(Node * linkedNode);
    virtual void remove(Node * linkedNode);
};

} /* namespace std */

#endif /* LIST_H_ */

Node.h

#ifndef NODE_H_
#define NODE_H_

#include "Data.h"

namespace std {

class Node {
public:
    Node();
    Node(Data * aItem);
    virtual ~Node();
    virtual Node* getNext();
    virtual void setNext(Node * linkedNode);
    virtual Data* getData();
    virtual void setData(Data* item);
private:
    Node *next;
    Data *item;
};

} /* namespace std */

#endif /* NODE_H_ */

Answer 1

You're defining List inside the std namespace, and forward declaring it outside of that. When you do this, you're saying "there's a class called List in the global namespace", which of course there isn't.

What you need to express is "there's a class called List inside the std namespace", which can be done as follows:

namespace std {
    class List;
}

class Data {
    /* your class */
};

I would question why you'd want to define your own class inside of namespace std, but that's beside the point.