Access violation in doubly linked list

Question

I posted earlier and got good help with figuring out how to deep copy my doubly linked list. I am now having a problem with an access violation "0xC000000005" which I believe is being caused by an attempt to deference a null pointer. This is a homework assignment and I am new to C++ so I'd just like some help figuring out where I am going wrong, as opposed to someone just giving me working code.

Here is my interface, given to me by my professor. I can not modify it in any way.

#ifndef TEXTANALYZER_H
#define TEXTANALYZER_H

#include <iostream>
#include <string>

using namespace std;

class TextAnalyzer {
private:

/*
* Class: Node
*
* This class represents a node in a sorted doubly linked list that stores a
* list of words and their frequency of occurrency.
*/
class Node {
public:
string word;
int wordFrequency;
Node* previous;
Node* next;

Node(const string& word,
     const int wordFrequency,
     Node* const previous,
     Node* const next)
: word(word),
      wordFrequency(wordFrequency),
      previous(previous),
      next(next)
    {}
}; // end ListNode
/*********************************************************************/

Node* head;
Node* tail;


/*
* Releases all the memory allocated to the list.
*/
void releaseNodes();

/*
* Makes a deep copy of the object.
*/
void copyNodes(Node* const copyHead);

/*
* Returns a populated Node.
* Throws a bad_alloc exception if memory is not allocated.
*/
Node* createNode(const string& word,
             const int wordFrequency,
             Node* const previous,
             Node* const next);

public:
/* 
* Initializes head and tail, each to a dymmy node.
*/
TextAnalyzer();

/*
* Makes a deep copy of the object passed in.
* Calls copyNodes() to do the actual work.     
*/
TextAnalyzer(const TextAnalyzer& copyObject);

/* 
* Releases all the memory allocated to the object.
* Calls the releaseNodes() method to do the actual work.
*/
~TextAnalyzer();

/* 
* Makes a deep copy of the rhs object.
*/
TextAnalyzer operator =(const TextAnalyzer& assignObject);

/*
* Inserts the word in a sorted order into the list. 
*
* If no Node exists with that initial character, one is added in
* sorted order. If one does exist (same word), then the word frequency
* of that word is incremented by one.
*/
void insertWord(const string& word);

/*
* Returns a count of all the words in the list.
*/
int wordCount() const;

/* 
* Returns a count of all the words with the initial character.
*/
int wordCountWithInitialCharacter(const char startsWith);

/*
* Returns a description of the object. The string is formatted as:
* [A words:]
*     [<word>(<count>)]
*     [<word>(<count>)]
*     ...
*
* [B words:]
*     [<word>(<count>)]
*     [<word>(<count>)]
*     ...
*
*...
*/
string toString() const;

};

#endif 

Here is my class definition:

#include "textAnalyzer.h"
#include <string>
#include <iostream>
#include <sstream>

TextAnalyzer::Node* TextAnalyzer::createNode(const string& word, const int wordFrequency, 
Node* const previous, Node* const next)
{
return new Node(word, wordFrequency, previous, next);
}
void TextAnalyzer::releaseNodes()
{
Node* del = tail;

while(tail != NULL)
{
    tail = tail->previous;
    tail->next = del;
    delete del;
    del = tail;
}

delete [] head;
delete [] tail;

head = tail = del = NULL;
}

void TextAnalyzer::copyNodes(Node* const copyHead)
{
head = new Node(*copyHead);
Node* iter = head->next;

for(Node* np = copyHead->next; np != NULL; np = np->next)
{
iter->next = new Node(*np);
iter = iter->next;
}

iter = NULL;
}

TextAnalyzer::TextAnalyzer():head(createNode("0",0,NULL,NULL)),tail(head)
{}

TextAnalyzer::TextAnalyzer(const TextAnalyzer& copyObject)
{
copyNodes(copyObject.head);
}

TextAnalyzer::~TextAnalyzer()
{
releaseNodes();
}

TextAnalyzer TextAnalyzer::operator=(const TextAnalyzer& assignObject)
{
return TextAnalyzer(assignObject);
}

void TextAnalyzer::insertWord(const string& word)
{
Node* iter = head->next;

while(iter != NULL)
{
if(iter->word == word)
    iter->wordFrequency++;
else if(iter->word[0] == word[0] && iter->next != NULL)
{
    Node* temp = iter->next;
    iter->next = createNode(word, 1, iter, temp);
    iter = iter->next;
    temp->previous = iter;

    temp = NULL;
}
else if(iter->word[0] == word[0] && iter->next == NULL)
{
    iter = createNode(word, 1, tail, NULL);
    tail = iter;
}
else
    iter = iter->next;
}

iter = NULL;
}

int TextAnalyzer::wordCount() const
{
Node* iter = head->next;
int count = 0;

while(iter != NULL)
count++;

return count;
}

int TextAnalyzer::wordCountWithInitialCharacter(const char startsWith)
{
Node* iter = head->next;
int count = 0;

for(int i = 0; i < wordCount(); i++)
{
if(startsWith == iter->word[0])
    count++;

iter->previous = iter;
iter = iter->next;
}

iter = NULL;

return count;
}

string TextAnalyzer::toString() const
{
Node* iter = head->next;
string desc = "List of words: \n";
ostringstream convert;

for(int i = 0; i < wordCount(); i++)
{
convert << iter->word[0] << " words:\n"
        << iter->word    << "(" 
        << iter->wordFrequency
        << ")\n";
iter->previous = iter;
iter = iter->next;
}

iter = NULL;

return desc + convert.str();
}

The problem occurs in my releaseNodes() method according to the debugger. Ive added a comment to it to point out the specific line it occurs on:

void TextAnalyzer::releaseNodes()
{
Node* del = tail;

while(tail != NULL)
{
    tail = tail->previous; //debugger flags this line when error occurs
    tail->next = del;
    delete del;
    del = tail;
}

delete [] head;
delete [] tail;

head = tail = del = NULL;
}

Im not sure what is causing the access violation, but like I said I am new to C++. Any and all help is appreciated.

Answer 1

In your copyNodes you need to set the previous pointer also

Answer 2

In addition to billz's answer, there's something fishy with your deletion loop. The way I understand it, you want to delete your list back to front. What lead you to the loop you have now? It seems that a more natural approach would work like: Maintain a curr pointer, starting at tail. Then loop: Decrement curr, delete curr->next. Stop when curr is head. Then delete head.

Answer 3

You do not allocate head and tail with new [], so you do not delete them by delete []

delete [] head;
delete [] tail;

Should be:

delete head;
delete tail;

Mix new with delete [] is undefined behavior.