C++：尝试将新节点添加到链表会产生 "Thread 1: EXC_BAD_ACCESS (code=1, address=0x0)" 错误

Question

我正在为创建和操作链表的家庭作业编写程序。我在 Node.cpp 中遇到 Node::SetData 函数的 "EXC_BAD_ACCESS" 错误，以及 List.cpp 中 List::Add_End 中的一行（特别是 "current->SetData(data);" ) 和 main.cpp 中的一行出于某种原因（特别是“// Add_End 列表中的节点”）。我假设一旦修复了 Node::SetData 错误，这些其他错误就会自行解决。

通过 Stack Overflow 和 Google 搜索后，我无法确定为什么会出现此错误。我认为这个问题 (New to C++, "EXC_BAD_ACCESS" error I don't understand) 会有所帮助，但我仍然遇到问题。

我犯了什么编码错误？

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

第一次调用 current->SetData 时（见下文）current 为 NULL，因此在访问它时出现页面错误（页面错误是现代操作系统在您尝试访问未分配内存时给您的错误。在wndows 术语通常是访问冲突。)

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

我设法更正了代码，所以我会解释我所做的，以防其他人遇到同样的问题。

ALTERATION: 在解释修复之前，让我先解释一下我所做的更改。链表的最后一个节点本身可以保存一个数据值，而不仅仅是 NULL（即最后一个节点的 data 不需要是 NULL，而是它的 next应该是 NULL)，所以我认为这样会更好。代码在每个重要的地方都反映了这一点，例如 List::Add_End(int data) 函数。

修复： 我修改了 List 构造函数以创建列表的头节点。因此，链表总是至少有一个节点，即使链表为空。稍后我将解释程序如何区分空列表和非空列表。

这里是原始构造函数：

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

这是新的构造函数：

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

        return;
    }

为什么要修改？ 据我所知，我遇到了 EXC_BAD_ACCESS 错误，因为 List::Add_End(int data) 函数试图操作链表的head 好像它是一个节点对象，但实际上它不是。（我相信这就是 marom 在他对这个问题的回答中的意思。）这就是为什么我改变了编码，使列表始终包含一个头节点，即使列表为空也是如此。

区分空列表和非空列表。 我更改了 Node 构造函数以将 data 设置为整数 -1122334455，而不是 NULL 就像我原来做的那样。因此，如果列表为空，则 head->Data()（即头节点的 data）为 -112233455 和 head->Next()（即头节点的 next）是 NULL。 这种方法的缺点是不可能有包含整数 -1122334455 的单项列表，但我认为可能不需要这个数字。只要列表至少有两项，head->Data() 可以是 -1122334455.

新代码： 代码的其余部分反映了这些修改。由于我只对 List.cpp 和 Node.cpp 进行了重大更改，因此我在下面仅复制了它们。其他三个程序文件基本没有变化。仅供参考，有许多多余的 return 和 this 我懒得删除。

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;
    }

C++：尝试将新节点添加到链表会产生 "Thread 1: EXC_BAD_ACCESS (code=1, address=0x0)" 错误

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