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

首次调用current-> setData（请参见下文）电流为null，因此访问它时您会得到页面故障（页面故障是现代OS的错误，如果您尝试尝试，请给您要访问未分配的内存。在WNDOWS下，该术语通常是访问违规的。）

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;
}

我设法纠正了代码，所以我会解释我所做的，以防其他人遇到同样的问题。

更改：在解释修复程序之前，让我解释一下我所做的更改。链接列表的最后一个节点可以保存数据值本身，而不仅仅是NULL（即，最后一个节点的data不需要是NULL，而且其next应该是NULL），所以我认为这会更好。该代码在重要的每个位置都反映了这一点，例如List::Add_End(int data)函数。

修复程序：我修改了List构造函数以创建列表的头节点。因此，即使列表为空，链接列表也将始终具有至少一个节点。我将解释程序以后如何辨别空白和非空的列表。

这是原始构造函数：

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

这是新的构造函数：

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

为什么要进行此修改？据我所知，我遇到了exc_bad_access错误，因为List::Add_End(int data)函数试图操纵链接的列表的head，就像是节点对象一样不是。（我相信这是Marom对这个问题的回答的含义。）这就是为什么我更改编码以使列表始终包含一个头节点的原因，即使列表为空。

在空和非空列表之间进行辨别。我将Node构造函数更改为将data设置为整数-1122334455，而不是像我最初一样的NULL。因此，如果列表为空，则head->Data()（即，Head Node的data）为-112233455和head->Next()（即，Head Node的next）是NULL。这种方法的不利之处在于，不可能拥有包含整数-1122334455的一个项目列表，但我认为这个数字可能不需要。只要列表至少有两个项目，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，因此我仅在下面重现它们。其他三个程序文件本质上是没有变化的。仅供参考，有许多冗余的return'S和this'我没有费心删除。

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;
    }