Seg Fault和Pointer问题

Question

Been working on this program for a couple of days in class and I keep getting seg faults. I can comment out the part of code that I think is causing it and I stop getting the fault but then my print function doesn't work. I am picking up the c++ book tomorrow to help.

//linkedList.cpp
//Declaration of main and menu functions
//Programmer: Ronnie Warden
//Date: 2.15.15

#include<iostream>
#include "link.h"
using namespace std;

int main()
{
    Node *list = new Node;
    int delNode;
    int findNode;
    int choice = 1;
    list->next = NULL;

    while (choice != 5)
    {
        choice = menu();

        if (choice == 1)
            *list = insertNode(list);

        if (choice == 2)
        {   
            cout << "Enter the ID you wish to delete: ";
            cin >> delNode;
            *list = deleteNode(list, delNode);
        }

        if (choice == 3)
            printNode(list);

        if (choice == 4)
        {
            cout << "Enter the ID you are searching for: ";
            cin >> findNode;
            *list = searchNode(list, findNode);
        }

    if (choice < 1 || choice > 5)
        cout << "Invalid choice! Please try again." << endl;

}

return 0;
}

int menu()
{
    int choice = 1;

         cout << "1. Insert Node" << endl;
         cout << "2. Delete Node" << endl;
         cout << "3. Print List" << endl;
         cout << "4. Search List" << endl;
         cout << "5. Quit" << endl;
         cin >> choice;

return choice;
}

This following part is the one throwing the seg fault.

//linkFun.cpp
//Declaration of functions used to manage the linked list
//Programmer: Ronnie Warden
//Date: 2.10.15

#include<iostream>
#include "link.h"
using namespace std;


/*************************************************************************************************************
Function: createNode
Parameters: No parameters
Return Type: Pointer to new node
Task: Create a node with "new" operator. Get a student data from keyboard and assign to members of the node.
*************************************************************************************************************/

Node createNode()
{
Node *newNode = new Node;

    cout << "Last Name?" << endl;
    cin >> newNode->lastName;
    cout << "First Name?" << endl;
    cin >> newNode->firstName;
    cout << "ID?" << endl;
    cin >> newNode->idNumber;

return *newNode;
}

/**************************************************************************************************************
Function: insertNode
Parameters: Pointer to the linked list
Return Type: Pointer to the linked list
Task: insert a new node to the linked list sorted by student's ID number. If ID is already in list notify user 
***************************************************************************************************************/

Node insertNode(Node *list)
{
    Node *newNode = new Node;
    Node *tmp = new Node;
    *newNode = createNode();
    int id = newNode->idNumber;

    if (list == NULL)           //Insert in empty list
        list->next = newNode;
    else
    {
         *tmp = searchNode(list, id);

         if (tmp->idNumber == newNode->idNumber)
             {
                cout << "ID number already in list! Please try again with a different ID number." << endl;
                insertNode(list);
        }

    if (list != NULL)
    {
        Node *tmp = list;

         if (tmp->idNumber > newNode->idNumber)     //Insert as first
         {
             newNode->next = tmp;
            list = newNode;
        }
    while (tmp->idNumber < newNode->idNumber)   //Search for insertion point
    {   
                if (tmp->next == NULL)        //Insert at end
                            tmp->next == newNode;

        tmp = tmp->next;

        if (tmp->idNumber < newNode->idNumber && tmp->next->idNumber > newNode->idNumber && tmp->next != NULL)  //Insert in-between
        {
            newNode->next = tmp->next->next;
            tmp->next = newNode;
        }

        }
    }
}
return *list;
}

 /***************************************************************************************************************
Function: searchNode
Parameters: Pointer to the linked list, student ID number
Return Type: Pointer to the node with matched ID number
Task: Search the linked list by student Id number. Notify user if ID is not found
***************************************************************************************************************/

Node searchNode(Node *list, int id)
{
    Node *missing = new Node;

    if (list->idNumber == id)   //Checks if first node matches id number
         return *list;
    else 
    {
        Node *tmp = new Node;   //creates temporary pointer to scroll through list
        while (tmp->idNumber != id)
       {    
            if (tmp->next == NULL)  //checks if number is missing returns sentinel if not found
            {
                missing->idNumber = 9999;   
                return *missing;
            }

             tmp = tmp->next;
         }
     return *tmp;

     }
 }

/***************************************************************************************************************
Function: deleteNode
Parameters: Pointer to the linked list, student ID number
Return Type: Pointer to the list
Task: Delete a node from the list. Notify user if no node matches
***************************************************************************************************************/

Node deleteNode(Node *list, int id)
{   
    Node *tmp = new Node;
    *tmp = searchNode(list, id);    
return *list;

}

/***************************************************************************************************************
Function: printNode
Parameters: Pointer to the linked list
Return Type: None
Task: Display the linked list
***************************************************************************************************************/

void printNode(Node *list)
{
    Node *tmp = new Node;
    tmp = list;

    while (tmp != NULL)
    {
        cout << tmp->lastName << endl;
        cout << tmp->firstName << endl;
        cout << tmp->idNumber << endl;
        tmp = tmp->next;
    }
}

If I comment out the entire else statement in the insertNode function I stop getting the seg fault but then when I call to print it prints 3 blank lines and then 0. Any help would be appreciated, and if it comes up, the reason I didnt use classes was because I am not allowed to, it has to be done in structured data type.

#ifndef LINK_H
#define LINK_H

struct Node {
    char lastName[20];
    char firstName[20];
    int idNumber;
    Node *next;
};

int menu();
Node createNode();
Node insertNode(Node*);
Node searchNode(Node*, int);
Node deleteNode(Node*, int);
void printNode(Node*);

#endif

Answer 1

Your main problem is that you're not using pointers right - you're kind of half way between pointers and objects. (I'm not going to address the fact that this is a C style program in C++)

For example: Your createNode() method returns a Node object. It does this by creating a new Node on the heap and then returning that node's contents. This means that you've actually got a memory leak. No one has remembered the pointer to the node you put in the heap and so it leaks.

My first step would be to make createNode() return a Node*

Node* createNode()
{
    Node *newNode = new Node;

    // All your cout/cin stuff

    return newNode;
}

I'd follow the same pattern with the other methods - use pointers rather than objects. Take searchNode() as an example. Because you're using objects, you need a special "Not Found" node. If you use pointers, you can return NULL (preferable 0 in C++) for the "not found case". You also don't need to new and temporary variables.

Node* searchNode(Node* list, int id)
{
    while(list != 0) {
        if (list->id == id) {
            return list;
        }
        // This only impacts the local list. It doesn't
        // permanently change the value of the list
        // passed in.
        list = list->next;
    }
    return 0;
}

For bonus points on searchNode() : Since it is a sorted list, you can stop searching as soon as you find an id that is greater than the one you are looking for.

Now that you've done that, your insert method is a bit simpler:

1. Get a new node from the user
2. Check if it exists - if so delete the node you just created and give error
3. If not, search the list and find where to insert your new node.