Question

As a requirement of an assignment for a data structures class, I have to get the following class hierarchy to work: http://www.brpreiss.com/books/opus4/

The source code is also provided, and right now I am simply trying to get stuff to compile. This has required re-organizing class definitions into their respective header files, moving template implementation into .inc files, and finishing bits of code which were not implemented. I have been making progress, but I am stuck on the following error (compiled with VC++):

1>main.obj : error LNK2019: unresolved external symbol "public: virtual __thiscall Iterator::~Iterator(void)" (??1Iterator@@UAE@XZ) referenced in function "public: virtual __thiscall NullIterator::~NullIterator(void)" (??1NullIterator@@UAE@XZ)

I have tried all of the usual solutions (eliminating redundant include statements, cleaning the project and recompiling, etc.) and am not sure where to go. As was previously discussed, the consensus here seems to be that this code-base is really poorly designed. Nonetheless, the requirement of this assignment is to get this code working, and I am into deep to scrap it all together and start from scratch. If it

Here is the iterator class definition in iterator.h

#ifndef ITERATOR_H
#define ITERATOR_H

#include "object.h"

class Iterator
{
public:

    virtual ~Iterator ();
    virtual void Reset () = 0;
    virtual bool IsDone () const = 0;
    virtual Object& operator * () const = 0;
    virtual void operator ++ () = 0;
};

class NullIterator : public Iterator
{
public:

    NullIterator () {}
    void Reset () {}
    bool IsDone () const { return true; }
    Object& operator * () const { return NullObject::Instance(); }
    void operator ++ () {}
};

#endif

These are all of the other header files which are associated with iterator:

#ifndef CONTAINER_H
#define CONTAINER_H

#include "object.h"
#include "visitor.h"
#include "iterator.h"
#include "ownership.h"

class Container : public virtual Object, public virtual Ownership
{
protected:

    unsigned int count;
    Container () : count(0) {}

public:

    virtual unsigned int Count () const { return count; }
    virtual bool IsEmpty () const { return Count () == 0; }
    virtual bool IsFull () const { return false; }
    //virtual HashValue Hash () const;
    virtual void Put (ostream&) const;
    virtual Iterator& NewIterator () const { return *new NullIterator (); }

    virtual void Purge () = 0;
    virtual void Accept (Visitor&) const = 0;
};

#endif

Stack and Queue also inherit from Container, but it appears only stack makes use of Iterator:

#ifndef STACK_H
#define STACK_H

#include "linkList.h"
#include "container.h"

class Stack : public virtual Container
{
public:

    virtual Object& Top () const = 0;
    virtual void Push (Object&) = 0;
    virtual Object& Pop () = 0;
};

class StackAsLinkedList : public Stack
{
    LinkedList<Object*> list;

    class Iter;

public:

    StackAsLinkedList () : list() {}
    ~StackAsLinkedList() { Purge(); }

    //
    // Push, Pop and Top
    //
    void Push(Object& object);
    Object& Pop() override;
    Object& Top() const override;

    int CompareTo(Object const& obj) const;

    //
    // purge elements from, and accept elements onto, the list
    //
    void Purge();
    void Accept (Visitor&) const;

    friend class Iter;
};

class StackAsLinkedList::Iter : public Iterator
{
    StackAsLinkedList const& stack;
    ListElement<Object*> const* position;

public:

    Iter (StackAsLinkedList const& _stack) : stack(_stack) { Reset(); }

    //
    // determine whether iterator is pointing at null
    //
    bool IsDone() const { return position == 0; }

    //
    // overloaded dereference and increment operator
    //
    Object& operator*() const;
    void   operator++();

    void Reset() { position = stack.list.Head(); }
};

#endif

If anyone has some insight, it would be much appreciated. I have been trying to solve this one error for the last few hours and haven’t made any progress!

Answer 1

The error is a linking error which tells you that you have not provided a definition for the destructor:

virtual ~Iterator();

Which is being called through:

NullIterator::~NullIterator(void)

because NullIterator derives from Iterator class.

Solution is You should provide the definition for the Base class destructor.