删除功能C的读取访问冲突异常

Question

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

class Person{
public:
    string name;
    int age, height, weight;

    Person(string name = "empty", int age = 0, int height = 0, int weight = 0) {
        this->name = name;
        this->age = age;
        this->height = height;
        this->weight = weight;
    }
    Person operator = (const Person &P) {
        name = P.name;
        age = P.age;
        height = P.height;
        weight = P.weight;

        return *this;
    }

    friend ostream& operator<<(ostream& os, const Person& p);
};

ostream& operator<<(ostream& os, Person& p) {
    os << "Name: " << p.name << "   " << "Age: " << p.age << "     " << "Height: " << p.height << " " << "Weight: " << p.weight << "\n";
    return os;
};

class Node {
public:
    Person* data;
    Node* next;
    Node(Person*A) {
        data = A;
        next = nullptr;
    }
};

class LinkedList {
public:
    Node * head;
    LinkedList() {
        head = nullptr;
    }

    void InsertAtHead(Person*A) {
        Node* node = new Node(A);
        node->next = head;
        head = node;
    }
    void InsertAtEnd(Person*A) {
        if (head == nullptr) {
            InsertAtHead(A);
        }
        else {
            Node* node = new Node(A);
            Node* temp = head;
            while (temp->next != nullptr) {
                temp = temp->next;
            }
            temp->next = node;
        }
    }
    void InsertAtPosition(Person*A, int pos) {
        if (head == nullptr) {
            InsertAtHead(A);
        }
        else {
            Node* node = new Node(A);
            Node* temp = head;
            for (int i = 1; i < pos - 1; i++) { temp = temp->next; }
            node->next = temp->next;
            temp->next = node;
        }
    }
    void DeleteByValue(string search_name) {
        Node* temp = head;
        while (temp != nullptr) {
            if (temp->data->name == search_name) {
                delete(temp);
            }
            else {
                temp = temp->next;
            }
        }
        cout << "No person with that name was in the list" << endl;
    }

    void DeleteFromHead() {
        if (head != nullptr) {
            Node* temp = head;
            head = head->next;
            delete temp;
        }
    }
    void DeleteFromEnd() {
        Node* prev = nullptr;
        Node* temp = head;
        if (head == nullptr) { cout << "Nothing to delete" << endl; }
        else if (head->next == nullptr) { DeleteFromHead(); }
       else {
            while (temp->next != nullptr) {
                prev = temp;
                temp = temp->next;
            }
            prev->next = nullptr;
            delete temp;
        }
    }
    void DeleteAtPosition(int pos) {
        Node* prev = nullptr;
        Node* temp = head;
        if (head == nullptr) { cout << "Nothing to delete" << endl; }
        else if (pos == 1) { DeleteFromHead(); }
        else {
            for (int i = 1; i < pos; i++) {
                prev = temp;
                temp = temp->next;
            }
            prev->next = temp->next;
            delete temp;
        }
    }
    void UpdateAtPosition(Person*A, int pos) {
        if (head == nullptr) { cout << "No element in the list"; return; }
        if (pos == 1) { head->data = A; }
        else {
            Node* temp = head;
            for (int i = 1; i < pos; i++) {
                temp = temp->next;
            }
            temp->data = A;
        }
    }

    void Print() {
        Node* temp = head;
        while (temp != nullptr) {
            cout << *(temp->data);
            temp = temp->next;
        }
        cout << endl;
    }
};
int main() {
    LinkedList* list = new LinkedList();
    Stack* stack = new Stack(3);
    DynamicStack* dstack = new DynamicStack();


    cout << "Linked List" << endl;
    cout << "-----------" << endl;
    list->InsertAtHead(new Person("Jeremy", 22, 70, 145));                  list->Print();
    list->InsertAtHead(new Person("Samantha", 20, 63, 115));                list->Print();
    list->InsertAtEnd(new Person("Chris", 19, 70, 200));                    list->Print();
    list->DeleteByValue("Chris");                                           list->Print();
    list->InsertAtPosition(new Person("Grace", 15, 64, 150), 3);            list->Print();
    list->InsertAtPosition(new Person("Robert", 15, 67, 160), 4);           list->Print();
    list->DeleteFromHead();                                                 list->Print();
    list->DeleteFromEnd();                                                  list->Print();
    list->DeleteAtPosition(2);                                              list->Print();
    list->UpdateAtPosition(new Person("Jeremy", 23, 70, 155), 1);           list->Print();
    cout << endl;
    cout << endl;
    system("pause");
}

I am new to C++ and I am trying to create a function for my linked list class that will Delete a Person object by the Persons name. I know this isn't showing the rest of the class, but I know it is all working and the error lies within this function. When I try and run the program there is an exception being thrown saying "Read Access Violation" on the line "delete(temp);". I am confident that I need to move the rest of the nodes back and make another node such as "prev" to store temp in before deleting it, but I have tried a lot and as I said I am new. Can anyone tell me what I need to add to get this to work, and please explain why so I am learning from it. Thanks in advance!

Answer 1

After you find a match, you delete it and the loop keeps going since you never set temp to nullptr . temp now is no longer valid since it has just been deleted, however, you try to access the data it points to on the next iteration, producing undefined behavior, then you delete it, likely causing the crash.

To fix it, either break the loop after a match is found or keep track of the next node before you delete temp.

Also, you should not have an output statement in the delete function since another person or you in the future won't expect a function that deletes elements to print stuff to the screen.

Answer 2

In your

void DeleteByValue(string search_name) {
    Node* temp = head;
    while (temp != nullptr) {
        if (temp->data->name == search_name) {
            delete(temp);
        }
        else {
            temp = temp->next;
        }
    }
    cout << "No person with that name was in the list" << endl;
}

once you find a match you just delete the memory but there is more to it, you need to handle both the previous node as well as head node if the name is found in the first node.

something like this

    Node* temp = head;
    Node* prev = nullptr;
    while (temp != nullptr) {
        if (temp->data->name == search_name) {
            if (prev != nullptr) {
              prev->next = temp->next;
            }
            else {
               head = temp->next;
            }
            delete temp;
            temp = nullptr;  
        }
        else {
            prev = temp;
            temp = temp->next;
        }
    }