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POAccess violation in doubly linked list
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copied!<p>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.</p> <p>Here is my interface, given to me by my professor. I can not modify it in any way.</p> <pre><code>#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 </code></pre> <p>Here is my class definition:</p> <pre><code>#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(); } </code></pre> <p>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:</p> <pre><code>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; } </code></pre> <p>Im not sure what is causing the access violation, but like I said I am new to C++. Any and all help is appreciated.</p>

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