C++双向链表的建立（无头节点）

#ifndef doublychain_
#define doublychain_
#include<iostream>
#include<algorithm>
#include"chainNode.h"

using namespace std;
template<typename T>
class doublychain
{
public:
doublychain();
~doublychain();
void insert(const int theIndex, const T& theElement);
void erase(int theIndex);
void output(ostream& out);
protected:
chainNode<T>*firstNode;
chainNode<T>*lastNode;
int listSize;
};
template<typename T>
doublychain<T>::doublychain()
{
firstNode = lastNode = NULL;
listSize = 0;
}

template<typename T>
doublychain<T>::~doublychain()
{
while (firstNode != NULL)
{
chainNode<T>*next = firstNode->next;
delete firstNode;
firstNode = next;
}
delete firstNode;
}

template<typename T>
void doublychain<T>::insert(const int theIndex, const T& theElement)
{
if (theIndex == 0)
{
firstNode = new chainNode<T>(theElement, firstNode);
lastNode = firstNode;
firstNode->pre = NULL;
lastNode->next = NULL;
}
else {
chainNode<T>*p = firstNode;
chainNode<T>*q;
for (int i = 0; i < theIndex -1; ++i)
p = p->next;
p->next = new chainNode<T>(theElement, p->next);
q = p->next;
q->pre = p;
if (theIndex ==listSize)
lastNode = p->next;
}
++listSize;
}
template<typename T>
void doublychain<T>::erase(int theIndex)
{
chainNode<T>*deleteNode;
chainNode<T>*q;
if (theIndex = 0)
{
deleteNode=firstNode;
q = firstNode->next;
firstNode = firstNode->next;
q->pre = NULL;

}
else {
chainNode<T>*p = firstNode;
for (int i = 0; i < theIndex - 1; i++)
p = p->next;
deleteNode=p->next;
q = p->next->next;
p->next = p->next->next;
q->pre = p;
if (deleteNode == lastNode)
lastNode = p;

}
delete deleteNode;
--listSize;
}
//对双向链表进行测试
template<typename T>
void doublychain<T>::output(ostream& out)
{
/*for (chainNode<T>*p = firstNode; p != NULL; p=p->next)
cout << p->element << " ";*/
for (chainNode<T>*p = lastNode; p !=NULL; p = p->pre)
cout << p->element << " ";
}
#endif