C ++：尝试将新节点添加到链接列表会产生“线程1：EXC_BAD_ACCESS（代码= 1，地址= 0x0）”错误

Question

I'm writing a program for a homework assignment that creates and manipulates a linked list. I am encountering an "EXC_BAD_ACCESS" error with the Node::SetData function in Node.cpp, as well as with a line in List::Add_End in List.cpp (specifically "current->SetData(data);") and a line in main.cpp for some reason (specifically "// Add_End nodes to the list"). I assume that once the Node::SetData error is fixed, these other errors will resolve themselves.

After searching through Stack Overflow and Google, I cannot determine why this error is occurring. I thought this question ( New to C++, "EXC_BAD_ACCESS" error I don't understand ) would help, but I'm still having issues.

What coding error(s) have I made?

main.cpp

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

#include "List.h"

int main()
{
    // New list
    List list;
    Node *answer;

    // Add_End nodes to the list
    list.Add_End(111);
    list.Print();
    list.Add_End(222);
    list.Print();
    list.Add_End(333);
    list.Print();
    list.Add_End(444);
    list.Print();
    list.Add_End(555);
    list.Print();

    // Delete nodes from the list
    list.Delete(444);
    list.Print();
    list.Delete(333);
    list.Print();
    list.Delete(222);
    list.Print();
    list.Delete(555);
    list.Print();
    list.Delete(111);
    list.Print();

    cout << "Testing Add_Front: and others" << endl;
    list.Add_Front(888);
    list.Print();
    list.Add_Front(999);
    list.Print();
    list.Add_Front(49);
    list.Print();

    cout << "Checking find function" << endl;
    answer = list.Find(888);
    cout << "Value for node returned by find function call with 888 is " << answer->Data() << "." << endl;
    cout << "Checking find function" << endl;
    answer = list.Find(999);
    cout << "Value for node returned by find function call with 888 is " << answer->Data() << "." << endl;
    cout << "Checking find function" << endl;
    answer = list.Find(49);
    cout << "Value for node returned by find function call with 888 is " << answer->Data() << "." << endl;
    cout << "Call find function with value not in list." << endl;
    answer = list.Find(7);
    if (answer == NULL)
    {
        cout << "returned null pointer since 7 not found" << endl;
    }
    else
    {
        cout << "in else of answer == NULL where Value for node returned by find function call with 7 is " << answer->Data() << "." << endl;
    }

    cout << "testing delete_front: " << endl;
    list.Delete_Front();
    list.Print();
    cout << "testing delete_end: " << endl;

    list.Delete_End();
    list.Print();

    return 0;
}

List.h

#ifndef LIST_H
#define LIST_H

#include <cstddef>

#include "Node.h"

class List
{
private:
    Node* head;

public:
    List();
    void Add_End(int data);
    void Delete(int data);
    void Delete_Front();
    void Add_Front(int data);
    void Delete_End();
    Node* Find(int data);
    void Print();    
};

#endif

List.cpp

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

#include "List.h"

List::List()
{
    head = NULL;
    return;
}

void List::Add_End(int data)
{
    Node* current;
    Node* newEnd = new Node();

    for (current = head; current != NULL; current = current->Next())
    {}
    current->SetData(data);
    current->SetNext(newEnd);
    newEnd->SetData(NULL);
    newEnd->SetNext(NULL);

    return;
}

void List::Delete(int data) {
    /*
     FILL IN CODE (will do later)
     */


    return;
}

void List::Delete_Front()
{
    /*
     FILL IN CODE (will do later)
     */

    return;
}

void List::Add_Front(int data)
{
    Node* newNode = new Node();
    newNode->SetData(data);
    newNode->SetNext(head);
    head = newNode;
    return;
}

void List::Delete_End()
{
    if (head == NULL)
    {
        cout << "List has no member so cannot delete end" << endl;
        return;
    }

    // check if one in length
    if (head->Next() == NULL)
    {
        head = NULL;
        return;
    }
    // 2 or greater in length

    Node* current;
    Node* prev;
    prev = head;
    for (current = head->Next(); current->Next() != NULL; current = current->Next())
    {
        prev = current;
    }
    prev->SetNext(NULL);
    return;
}

Node* List::Find(int data)
{
    Node* current;

    for (current = head; current != NULL && current->Data() != data; current = current->Next())
    {}
    if(current == NULL)
    {
        cout << "Did not find " << data << "." << endl;
        return NULL;
    }
    else // found
    {
        cout << "Found " << data << "." << endl;
        return current;
    }
}

void List::Print()
{
    Node* current;
    for (current = head; current != NULL; current = current->Next())
    {
        cout << current->Data() << " ";
    }
    cout << endl;

    return;
}

Node.h

#ifndef NODE_H
#define NODE_H

class Node
{
private:
    int data;
    Node* next;

public:
    Node();
    void SetData(int aData);
    void SetNext(Node* aNext);
    int Data();
    Node* Next();
};

#endif

Node.cpp

#include <cstddef>

#include "Node.h"

Node::Node()
{
    this->SetData(NULL);
    this->SetNext(NULL);
    return;
}

void Node::SetData(int aData)
{
    this->data = aData;
    return;
}

void Node::SetNext(Node* aNext)
{
    this->next = aNext;
    return;
}

int Node::Data()
{
    return data;
}

Node* Node::Next()
{
    return next;
}

Answer 1

While calling current->SetData for the first time (see below) current is NULL and so you get page fault when accessing it (page fault is the error modern OSes give you if you try to access unallocated memory. Under wndows the term usually is Access violation.)

void List::Add_End(int data)
{
    Node* current;
    Node* newEnd = new Node();

    for (current = head; current != NULL; current = current->Next())
    {}
    current->SetData(data);
    current->SetNext(newEnd);
    newEnd->SetData(NULL);
    newEnd->SetNext(NULL);

    return;
}

Answer 2

I managed to correct the code, so I'll explain what I did in case someone else encounters the same problem.

ALTERATION: Before I explain the fix, let me explain a change I made. The last node of the linked list can hold a data value itself, not just NULL (ie, the last node's data does not need to be NULL , but its next should be NULL ), so I thought this would be better. The code reflects this in every location where it matters, such as the List::Add_End(int data) function.

---

THE FIX: I modified the List constructor to create a head node for the list. So, the linked list will always have at least one node, even if the list is empty. I will explain how the program discerns between empty and nonempty lists later.

Here is the original constructor:

    List::List()
    {
        head = NULL;
        return;
    }

Here is the new constructor:

    List::List()
    {
        Node* headNode = new Node();
        head = headNode;

        return;
    }

Why make this modification? As far as I can tell, I encountered the EXC_BAD_ACCESS error because the List::Add_End(int data) function tried to manipulate the linked list's head as if it were a node object, when actually it was not. (I believe this is what marom meant in his answer to this question.) This is why I altered the coding such that the list always contains a head node, even when the list is empty.

Discerning between empty and nonempty lists. I altered the Node constructor to set data to the integer -1122334455 , instead of NULL like I originally did. So, if the list is empty, then head->Data() (ie, the head node's data ) is -112233455 and head->Next() (ie, the head node's next ) is NULL . The downside to this approach is that it's impossible to have a one-item list containing the integer -1122334455 , but I figure this number is likely to be unneeded. As long as the list has at least two items, head->Data() can be -1122334455 .

---

NEW CODE: The rest of the code reflects these modifications. Since I only made significant changes to List.cpp and Node.cpp, I have reproduced only them below. The other three program files are essentially unchanged. FYI, there are many redundant return 's and this 's that I didn't bother to delete.

List.cpp

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

    #include "List.h"

    // -1122334455 is an arbitrary integer that is likely to never be needed by the user

    List::List()
    {
        Node* headNode = new Node();
        head = headNode;

        return;
    }

    Node* List::Add_End(int data)
    {
        // if list is empty (i.e., list has only head node with data == -1122334455 & next == NULL)
        if (head->Data() == -1122334455 && head->Next() == NULL)
        {
            head->SetData(data);

            return head;
        }
        // if list is nonempty
        else
        {
            Node* current;
            Node* newEnd = new Node();

            for (current = head; current->Next() != NULL; current = current->Next())
            {}
            current->SetNext(newEnd);
            newEnd->SetData(data);
            newEnd->SetNext(NULL);

            return newEnd;
        }
    }

    void List::Delete(int data)
    {
        Node* prev;
        Node* current;

        // if list is empty
        if (head->Data() == -1122334455 && head->Next() == NULL)
        {
            cout << "Cannot delete this datum because list is empty." << endl;
            return;
        }

        // if list contains 1 element
        if (head->Data() == data && head->Next() == NULL)
        {
            head->SetData(-1122334455);
            return;
        }
        else if (head->Data() != data && head->Next() == NULL)
        {
            cout << "Datum not found in list." << endl;
            return;
        }

        // if list contains 2 or more elements
        prev = head;

        for (current = head->Next(); current->Data() != data && current->Next() != NULL; current = current->Next())
        {
            prev = prev->Next();
        }
        if (current->Data() == data && current->Next() != NULL)
        {
            prev->SetNext(current->Next());
            delete current;
            return;
        }
        else if (current->Data() == data && current->Next() == NULL)
        {
            prev->SetNext(NULL);
            delete current;
            return;
        }
        else
        {
            cout << "Datum not found in list." << endl;
            return;
        }
    }

    void List::Delete_Front()
    {
        Node* origHead = head;
        Node* newHead = head->Next();

        head = newHead;
        delete origHead;

        return;
    }

    void List::Add_Front(int data)
    {
        // if list is empty
        if (head->Data() == -1122334455 && head->Next() == NULL)
        {
            head->SetData(data);
            return;
        }

        // if list is nonempty
        Node* newNode = new Node();
        newNode->SetData(data);
        newNode->SetNext(head);
        head = newNode;
        return;
    }

    void List::Delete_End()
    {
        if (head->Data() == -1122334455 && head->Next() == NULL)
        {
            cout << "List has no member so cannot delete end" << endl;
            return;
        }

        // check if one in length
        else if (head->Data() != -1122334455 && head->Next() == NULL)
        {
            head->SetData(-1122334455);
            return;
        }

        // 2 or greater in length
        else
        {
            Node* current;
            Node* prev;
            prev = head;
            for (current = head->Next(); current->Next() != NULL; current = current->Next())
            {
                prev = current;
            }
            prev->SetNext(NULL);
            return;
        }
    }

    Node* List::Find(int data)
    {
        Node* current;

        for (current = head; current != NULL && current->Data() != data; current = current->Next())
        {}
        if (current == NULL)
        {
            cout << "Did not find " << data << "." << endl;
            return NULL;
        }
        else // found
        {
            cout << "Found " << data << "." << endl;
            return current;
        }
    }

    void List::Print()
    {
        if (head->Data() == -1122334455 && head->Next() == NULL)
        {
            cout << "List is empty." << endl;
            return;
        }

        Node* current;
        for (current = head; current != NULL; current = current->Next())
        {
            cout << current->Data() << " ";
        }
        cout << endl;

        return;
    }

Node.cpp

    #include <cstddef>

    #include "Node.h"

    Node::Node()
    {
        // -1122334455 is an arbitrary integer that is likely to never be needed by the user
        this->SetData(-1122334455);
        this->SetNext(NULL);
        return;
    }

    void Node::SetData(int aData)
    {
        this->data = aData;
        return;
    }

    void Node::SetNext(Node* aNext)
    {
        this->next = aNext;
        return;
    }

    int Node::Data()
    {
        return this->data;
    }

    Node* Node::Next()
    {
        return this->next;
    }