是什么导致指针重置？
What's causing the pointer to reset?
我正在为类编写一个程序，该程序从模板类中创建一个链表，以保存有关学生、教师和管理的信息。学生、管理员和教员都继承了一个名为Person的类。我的问题是在我的列表类和RecordsOffice类中，当程序离开addToList函数时，头指针会重置它的数据，我看不到我刚刚添加到列表中的内容。
这是TemplatedList类
//Author:Angelo Todaro
//TemplatedList.h
//Header file for the TemplatedList class
#include "person.h"
#include "student.h"
#include "administrator.h"
#include "faculty.h"
using namespace std;
#ifndef TemplatedList_H
#define TemplatedList_H
template<class T>
struct Node{
T *data;
Node *next;
};
template <class T, typename T2>
class TemplatedList{
public:
/*Default Constructor
*Constructs an object of the TemplatedList class*/
TemplatedList();
/*Default Destructor
*Destructs an object of the TemplatedList class*/
~TemplatedList();
/*Accessor Method for head node
*Provides user access to the value of the head node*/
T getHead() const;
/*Modifier Method for head node
*Provides the user access to change the value of the head node*/
void setHead(T *newHead);
/*addToList
*Adds a Student to the list*/
void addToList(T *newStudent);
/*deleteFromList
*Deletes a Student from the list*/
void deleteFromList(T2 number);
/*printStudent
*prints the information of the specified student*/
void printSingle(T2 number);
/*printList
*prints the entire list of students*/
void printList();
private:
T *head;
};
#endif
/*Default Constructor
*Constructs an object of the TemplatedList class*/
template <class T, typename T2>
TemplatedList<T,T2>::TemplatedList(){
head = new T;
head->next=NULL;
head->data=NULL;
}
/*Default Destructor
*Destructs an object of the TemplatedList class*/
template <class T, typename T2>
TemplatedList<T,T2>::~TemplatedList(){
delete head;
head=NULL;
}
/*Accessor Method for head node
*Provides user access to the value of the head node*/
template <class T, typename T2>
T TemplatedList<T,T2>::getHead() const{
return head;
}
/*Modifier Method for head node
*Provides the user access to change the value of the head node*/
template <class T, typename T2>
void TemplatedList<T,T2>::setHead(T *newHead){
head=newHead;
}
/*addToList
*Adds a Person to the list*/
template <class T, typename T2>
void TemplatedList<T,T2>::addToList(T *newPerson){
T *curr=head;
T *prev=curr;
cout << "Adding ";
newPerson->data->print();
cout << endl;
if(head->next==NULL && head->data==NULL)//if list has not been initialized
{
head->data=newPerson->data;
head->data->print();
head->next=NULL;
}
//if new Person's M Number is less than the head's MNumber
else if(newPerson->data->getMNumber()<head->data->getMNumber())
{
newPerson->next=head->next;
head = newPerson;
}
else //all other cases
{
while(curr!=NULL &&
newPerson->data->getMNumber()>curr->data->getMNumber())
{
prev=curr;
curr=curr->next;
}
newPerson->next=curr;
prev->next= newPerson;
if(curr==NULL)
newPerson->next=NULL;
}
}
/*deleteFromList
*Deletes a Person from the list*/
template <class T, typename T2>
void TemplatedList<T,T2>::deleteFromList(T2 number){
T *curr=head;
T *prev=head;
if(head==NULL||head->next==NULL){
cout << "Can not delete Person (M" << number << "), NOT found!" << endl;
}
//if first Person is equal to the selected one to delete
else if(curr->data->getMNumber()==number)
{
prev->next=curr->next;
delete curr;
curr=NULL;
}
else
{
while(curr!=NULL &&
curr->data->getMNumber()!=number)
{
prev=curr;
curr=curr->next;
}
if(curr!=NULL)//If not at end of list
{
prev->next=curr->next;
delete curr;
curr = NULL;
}
else//there is no listing
{
cout << "Can not delete Person (M" << number << "), NOT found!" << endl;
}
}
cout << endl;
}
/*printPerson
*prints the information of the specified Person*/
template <class T, typename T2>
void TemplatedList<T,T2>::printSingle(T2 number){
T *curr=head;
T *prev=curr;
while(curr!=NULL && curr->data->getMNumber()!=number)
{
prev=curr;
curr=curr->next;
}
if(curr!=NULL)//if match is found
{
curr->data->print();
}
else//there is no listing
{
cout << "Can not print person (M" << number << "), NOT found!" << endl << endl;
}
}
/*printList
*prints the entire list of Persons*/
template <class T, typename T2>
void TemplatedList<T,T2>::printList(){
T *curr=head;
T *prev=curr;
while(curr!=NULL)
{
curr->data->print();
prev=curr;
curr=curr->next;
}
cout << endl;
}
以下是RecordsOffice类的实现
//Author:Angelo Todaro
//recordsOffice.cpp
//Implementation file for the RecordsOffice class
#include <iostream>
#include <fstream>
#include <cassert>
#include "recordsOffice.h"
/*Default Constructor
Creates an object of the RecordsOffice class*/
RecordsOffice::RecordsOffice(){
}
/*Default Destructor
*Destructs an object of the RecordsOffice class*/
RecordsOffice::~RecordsOffice(){
}
/*Accessor Method for List of Persons
*Provides the user with a way to retrieve m_List*/
TemplatedList<Node<Person>,int> RecordsOffice::getList() const{
return m_List;
}
/*Modifier Method for List of Persons
*provides a way for the user to modify m_List*/
void RecordsOffice::setList(TemplatedList<Node<Person>,int> newList){
m_List = newList;
}
/*parseCommands
*parses the command entered and calls appropriate function*/
void RecordsOffice::parseCommands(string fileName){
ifstream infile;//create new file stream
infile.open(fileName);//opens file
assert(infile);//check for valid file
char command;//stores the value of the current command
while(infile >> command)
{
if(command=='A')
{
string name, email, title;//temporary strings to hold values of Person
int mnumber;//temporary ints to hold values of Person
infile >> name >> mnumber >> email >> title;
Administrator admin = Administrator(name,mnumber,email,title);
Node<Person> *newper = new Node<Person>;
newper->data=&admin;
newper->next=NULL;
m_List.addToList(newper);
}
else if(command=='F')
{
string name, email, department;//temporary strings to hold values of Person
int mnumber;//temporary ints to hold values of Person
bool tenured;
infile >> name >> mnumber >> email >> department >> tenured;
Faculty fac= Faculty(name,mnumber,email,tenured,department);
Node<Person> *newper = new Node<Person>;
newper->data=&fac;
newper->next=NULL;
m_List.addToList(newper);
}
else if(command=='S')
{
string name, email, major;//temporary strings to hold values of Person
int mnumber, year;//temporary ints to hold values of Person
infile >> name >> mnumber >> email >> year >> major;
Student stu= Student(name,mnumber,email,year,major);
Node<Person> *newper = new Node<Person>;
newper->data=&stu;
newper->next=NULL;
m_List.addToList(newper);
}
else if(command=='D')
{
int mnum;//int to store mnumber
infile >> mnum;
cout << "Deleting M" << mnum << endl;
m_List.deleteFromList(mnum);
}
else if(command=='P')
{
int mnum;//int to store mnumber
infile >> mnum;
cout << "Printing M" << mnum << endl;
m_List.printSingle(mnum);
}
else if(command=='L')
{
cout << "Listing all Persons" << endl;
m_List.printList();
}
}
}
程序正在读取的文件包含
A Doe 4444 jdoe3@mtsu.com President
L
P 4444
F Johnson 2222 cjLittle@mtsu.com English 0
P 2222
S Reed 9999 reed@mtsu.com 1 History
P 9999
L
D 9999
D 2222
D 4444
newper->data总是指向局部变量，当局部变量超出范围时，它们实际上被释放了，newper->data不再指向有效数据。您需要通过new使用动态存储持续时间。
Administrator admin = Administrator(name,mnumber,email,title);
newper->data=&admin;
Faculty fac= Faculty(name,mnumber,email,tenured,department);
newper->data=&fac;
Student stu= Student(name,mnumber,email,year,major);
newper->data=&stu;
尝试：
Person *admin = new Administrator(name,mnumber,email,title);
newper->data = admin;
Person *fac = new Faculty(name,mnumber,email,tenured,department);
newper->data = fac;
Person * stu = new Student(name,mnumber,email,year,major);
newper->data=stu;
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