双重链接列表的深度复制

我在深度复制双链表时遇到问题。这是一项家庭作业，所以我想知道为什么我的代码不工作，而不是得到我不理解的工作代码。

这是我的课：

#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();
}

这是接口：

#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 occurrence.
 */
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 

我需要使用上面给出的界面。我的问题是，我在复制构造函数中收到一个错误，说"对象的限定符不兼容"或类似的话。我假设这是因为copyObject是常数。然而，我不知道该怎么做，有人能告诉我我在这里错过了什么吗？我是C++的新手，我对Java更有经验，所以这可能是我感到困惑的原因。

编辑：

感谢您的回复。我想我即将想出如何成功地进行深度复制。我已经更新了我的代码，以显示到目前为止我已经完成的内容。现在我已经编译了代码，我得到了一个新的错误。每次运行它时都会出现"未处理的异常0xc0000005"。我在谷歌上搜索了它，并认为这是由于试图取消引用空指针而导致的错误。调试器显示它是在我的releaseNodes()方法中抛出的。

void TextAnalyzer::releaseNodes()
{
Node* del = tail;
while(tail != NULL)
{
    tail = tail->previous; //error on this line
    tail->next = del;
    delete del;
    del = tail;
 }
delete [] head;
delete [] tail;
head = tail = del = NULL;
}

上面只是我的releaseNodes()方法，其中有一条注释显示调试器指出错误的来源。我想看看我的其余代码是否有效，因为我是C++的新手，很可能我的逻辑在其他地方也有缺陷，不幸的是，在这个错误得到解决之前，我无法测试任何东西。我仍在追踪我的代码，试图找出可能的原因。如果有人能给我指明正确的方向，我将不胜感激。