"lvalue required as left operand of assignment" 编写链表时出错

Question

我目前正在为我在学校上的课程学习一些 C++。我对左值和右值有基本的了解，但我无法确定为什么会收到编译器错误。

我正在创建一个单链表并且需要能够反转它。根据我的任务，我有两个 classes。第一个是节点，它只包含一个整数和一个指针。

 class Node {
  int data;
  Node *next;

  public:
    //Constructor
    Node(int d) {
      data = d;
      next = NULL;}

    //Set to next Node
    void SetNext(Node *nextOne) {
      next = nextOne;}

    //Returns data value
    int Data(){return data;}

    //Returns next Node
    Node *Next() {return next;}
};

然后我有一个链表 class，它有一个 header 指针，然后是一些用于添加、打印等列表的函数。

class LinkedList {
  Node *head;

  public:
    //Constructor
    LinkedList(){head = NULL;}

    void AddNode(int d) {
      //Create a new Node
      Node *newNode = new Node(d);

      //Create a temporary pointer
      Node *temp = head;

      //If there are already nodes in the list
      if(temp != NULL) {
        //Parse through to the end of the list
        while(temp->Next() != NULL) {
          temp = temp->Next();}
        //Point the last Node in the list to the new Node
        temp->SetNext(newNode);
      }

      //If adding as the first Node
      else{
        head = newNode;}
    }

    void PrintList() {
      //Temporary pointer
      Node *temp = head;

      //If there are no nodes in the list
      if(temp == NULL) {
        std::cout << "The list is empty" << std::endl;}

      //If there is only one node in the list
      if(temp->Next() == NULL) {
          std::cout << temp->Data() << std::endl;}

        //Parse through the list and print
      else {
        do {
          std::cout << temp->Data();
          temp = temp->Next();
        }
        while(temp != NULL);
      }
    }

    //Returns the number of nodes in the list
    int CountList() {
      //Temporary pointer
      Node *temp = head;
      //Counter variable
      int counter = 0;

      //If the list is empty
      if(temp == NULL) {
        return counter;}

      //Parse through Nodes counting them
      else {
        do {counter++;
          temp = temp->Next();
        }
        while(temp != NULL);
      }
      return counter;
    }

    //Reverses the list
    Node *ReverseList() {
      //Initially set to NULL then tracks the new head
      Node *marker = NULL;
      //Tracks the next one in the list
      Node *nextOne;

      //Sets the first Node to NULL and then sets the last Node to point to
      //the first one and rotates through the list pointing the last to the
      //first
      while(head != NULL) {
        nextOne = head->Next();
        head->Next() = marker;
        marker = head;
        head = nextOne;
      }
      //Setting the head back to the start again
      head = marker;
    }

};

其中一个函数应该反转列表。 ReverseList 函数中的 "head->Next() = marker;" 行在编译时导致 "lvalue required as left operand of assignment" 错误。

关于为什么会发生这种情况以及如何解决问题的任何见解？

提前致谢！

Answer 1

你的 Next() 函数 returns 一个指针，然后你这样做：

head->Next() = marker;

您正在更改指向标记的指针，而不是它指向的内容。要解决这个问题，您需要取消引用该指针：

*head->Next() = marker;

Answer 2

调用 Next() 的 return 是一个右值。由于你在class函数中，你不需要调用Next函数来获取私有next指针，你可以直接使用它。

head->next = marker;

Answer 3

您的下一个签名是：

Node *Next() {return next;}

这会在 return 复制下一个指针，因此它被视为右值而不是左值。

克服这个问题的一种方法是使用指向指针的指针：.

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

然后将其用作：

  int main()
  { 
  Node* marker=new Node(89);
  Node* nod=new Node(9);
  *(nod->Next())= marker;


  cout<<(nod->next)->data<<endl;
  cout << "Hello World" << endl; 

   return 0;
  }

这使得使用起来更加复杂。

"lvalue required as left operand of assignment" 编写链表时出错

"lvalue required as left operand of assignment" error writing a linked list

c++

compiler-errors

linked-list

lvalue

singly-linked-list