C++ 如何修复抛出的异常：写访问冲突。 这是空指针。 20号线

Question

*Create a simple linked list program to create a class list containing

class node {
        void *info;
            node *next;
public:
            node (void *v) {info = v; next = 0; }
            void put_next (node *n) {next = n;}
        node *get_next ( ) {return next;}
            void *get_info ( ) {return info;}
};

Be able to initially fill the list. Provide functions to insert/append nodes and remove nodes from the linked list. Be able to display the contents of the list. Write a little driver program with at least 5 values passed in (so that 5 nodes are created) as you insert/append, delete and display data, showing the programs operation. Output: While displaying the data, make sure you use an informational label–using the terms “insert”, “append”, “remove” and any other term which displays the action. All data that is displayed must be displayed in a way in which the mentor can easily read and understand.*

My code runs but it throws Exception thrown: write access violation. this was nullptr. on Line 20 when I debug it. In the output it also stops at Delete 04 of my list and outputs no further data after line 156. I am not sure how to fix this. I tried to force it to output the new list with another cout statement but that did not work. Any assistance would be appreciated. Entire code below.

#include <iostream>
#include <iomanip>
using namespace std;

class Node
{
    void* info;
    Node* next;
public:
    Node(void* v) { info = v; next = 0; }
    void put_next(Node* n)
    {
        next = n;
    }
    Node* get_next()
    {
        return next;
    }
    void* get_info()
    {
        return info;
    }
};

class List {
    Node* head;
public:
    List() { head = NULL; };
    void PRINT();
    void APPEND(void* info);
    void DELETE(void* info);
};

void List::PRINT() {

    Node* temp = head;

    if (temp == NULL)
    {
        cout << "Enter Values" << endl;
        return;
    }

    if (temp->get_next() == NULL)
    {
        cout << *(int*)temp->get_info();
        cout << " => ";
        cout << "Invalid" << endl;
    }
    else
    {
        while (temp != NULL)
        {
            cout << *(int*)temp->get_info();
            cout << " --> ";
            temp = temp->get_next();
        }

        cout << "Invalid" << endl;
    }
}

void List::APPEND(void* info) {

    Node* newNode = new Node(info);
    newNode->put_next(NULL);

    Node* temp = head;

    if (temp != NULL)
    {
        while (temp->get_next() != NULL)
        {
            temp = temp->get_next();
        }

        temp->put_next(newNode);
    }
    else
    {
        head = newNode;
    }
}

void List::DELETE(void* info) {

    Node* temp = head;

    if (temp == NULL)
    {
        cout << "Enter Values" << endl;
        return;
    }

    if (temp->get_next() == NULL)
    {
        if ((int*)(temp->get_info()) == info)
        {
            delete temp;
            head = NULL;
        }
    }
    else
    {
        Node* previous = NULL;
        while (temp != NULL)
        {
            if ((int*)(temp->get_info()) == info) break;
            previous = temp;
            temp = temp->get_next();
        }

        previous->put_next(temp->get_next());

        delete temp;
    }
}

int main()
{

    List list;

    int a = 04;
    int b = 11;
    int c = 12;
    int d = 15;
    int e = 29;
    int* Node1 = &a;
    int* Node2 = &b;
    int* Node3 = &c;
    int* Node4 = &d;
    int* Node5 = &e;

    cout << "Append: 04" << endl;
    list.APPEND(Node1);
    cout << "Append: 11" << endl;
    list.APPEND(Node2);
    cout << "Append: 12" << endl;
    list.APPEND(Node3);
    cout << "Append: 15" << endl;
    list.APPEND(Node4);
    cout << "Append: 29" << endl;
    list.APPEND(Node5);

    cout << endl << "Print List" << endl;
    list.PRINT();

    cout << endl << "Delete 04" << endl;
    list.DELETE(Node1);
    list.PRINT();
    cout << "Start New List" << endl;
    cout << endl << "Delete 12" << endl;
    list.DELETE(Node3);
    list.PRINT();

    return 0;
}

SOLUTION:

//Node.h

#pragma once
class Node
{
    void* info;
    Node* next;
public:
    Node(void* v) { info = v; next = 0; }
    void put_next(Node* n)
    {
        next = n;
    }
    Node* get_next()
    {
        return next;
    }
    void* get_info()
    {
        return info;
    }
};

//List.h

#pragma once
#include "Node.h"
#include <iomanip>

class List {
    Node* head;
public:
    List() { head = NULL; };
    void PRINT();
    void APPEND(void* info);
    void DELETE(void* info);
};

//List.cpp

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

void List::PRINT() {

    Node* temp = head;

    if (temp == NULL)
    {
        cout << "Enter Values" << endl;
        return;
    }

    if (temp->get_next() == NULL)
    {
        cout << *(int*)temp->get_info();
        cout << " => ";
        cout << "Invalid" << endl;
    }
    else
    {
        while (temp != NULL)
        {
            cout << *(int*)temp->get_info();
            cout << " --> ";
            temp = temp->get_next();
        }

        cout << "Invalid" << endl;
    }
}

void List::APPEND(void* info) {

    Node* newNode = new Node(info);
    newNode->put_next(NULL);

    Node* temp = head;

    if (temp != NULL)
    {
        while (temp->get_next() != NULL)
        {
            temp = temp->get_next();
        }

        temp->put_next(newNode);
    }
    else
    {
        head = newNode;
    }
}

void List::DELETE(void* info) {

    Node* temp = head;

    if (temp == NULL)
    {
        cout << "Enter Values" << endl;
        return;
    }

    if (temp->get_next() == NULL)
    {
        if ((int*)(temp->get_info()) == info)
        {
            delete temp;
            head = NULL;
        }
    }
    else
    {
        Node* previous = NULL;

        while (temp != NULL)
        {
            if ((int*)(temp->get_info()) == info) {

                // if node is head then changing the head
                if (temp == head)
                    head = temp->get_next();
                break;
            }
            previous = temp;
            temp = temp->get_next();
        }

        // skip this operation if node is head
        if (temp->get_next() != head)
            previous->put_next(temp->get_next());

        delete temp;
    }
}

//Main.cpp

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

int main()
{

    List list;

    int a = 04;
    int b = 11;
    int c = 12;
    int d = 15;
    int e = 29;
    int* Node1 = &a;
    int* Node2 = &b;
    int* Node3 = &c;
    int* Node4 = &d;
    int* Node5 = &e;

    cout << "Append: 04" << endl;
    list.APPEND(Node1);
    cout << "Append: 11" << endl;
    list.APPEND(Node2);
    cout << "Append: 12" << endl;
    list.APPEND(Node3);
    cout << "Append: 15" << endl;
    list.APPEND(Node4);
    cout << "Append: 29" << endl;
    list.APPEND(Node5);

    cout << endl << "Print List" << endl;
    list.PRINT();

    cout << endl << "Delete 04" << endl;
    list.DELETE(Node1);
    list.PRINT();
    cout << "Start New List" << endl;
    cout << endl << "Delete 12" << endl;
    list.DELETE(Node3);
    list.PRINT();

    return 0;
}

Answer 1

So whenever you are deleting first node, you missed to set the head to the next node of head .

Commented at the parts which are modified in the else statement of DELETE function

Code:

else
    {
        Node* previous = NULL;

        while (temp != NULL)
        {
            if ((int*)(temp->get_info()) == info){ 
             
            // if node is head then changing the head
            if (temp == head)
                head = temp->get_next();
                break;
            }
            previous = temp;
            temp = temp->get_next();
        }
        
        // skip this operation if node is head
        if (temp->get_next() != head)
        previous->put_next(temp->get_next());

        delete temp;
    }