添加新节点时，链表节点尾部不更新

Question

我正在玩一个链接列表类项目的指针，我不知道如何创建到新节点的链接。我有一个Linked List类，它包含像append这样的方法来操作数据结构。我希望这些节点是从csv文件中读取的出价。

当我从CSV加载所有数据时，我想

1. 创建一个新的出价
2. 将新的出价传递给append函数
3. 设置Bid对象的nextBid指针，并更新链接列表的尾部

我将不胜感激为每个出价对象创建新地址的任何指针，因为现在尾节点只'记得'第一个出价的地址。

Old Tail: 0x7ffeefbfee48
New Tail: 0x7ffeefbfee48
Old Tail: 0x7ffeefbfee48
New Tail: 0x7ffeefbfee48

我复制了下面的代码，省略了与从csv文件加载出价无关的部分：

#include <algorithm>
#include <iostream>
#include <time.h>

#include "CSVparser.hpp"

using namespace std;

// forward declarations
double strToDouble(string str, char ch);

// define a structure to hold bid information
struct Bid {
    string bidId; // unique identifier
    string title;
    string fund;
    double amount;
    Bid* nextBid; //each bid has a pointer that can point to another bid
    Bid() {
        amount = 0.0;
    }
};

class LinkedList {
    
private:
    // FIXME (1): Internal structure for list entries, housekeeping variables
    Bid* head;
    Bid* tail;
    
public:
    LinkedList();
    virtual ~LinkedList();
    void Append(Bid bid);
    void Prepend(Bid bid);
    void PrintList();
    void Remove(string bidId);
    Bid Search(string bidId);
    int Size();
};

LinkedList::LinkedList() {
    // FIXME (2): Initialize housekeeping variables
    head=nullptr; //initialize head to point to nothing
    tail=nullptr;
}

void LinkedList::Append(Bid bid) { //<---I'm having trouble with this method
    // FIXME (3): Implement append logic
    if (this->head==nullptr){ //first node in a linked list
        cout << "initialize head and tail" << endl;
        this->head=&bid; //point to the bid
        this->tail=&bid;
    }
    else {
        cout << "Old Tail: " << this->tail << endl;
        this->tail->nextBid=&bid; //this refers to bid
        this->tail=&bid; //update last bid
        cout << "New Tail: " << &bid << endl;
        this->tail->nextBid=nullptr; //set pointer after last bid to null
    }
}

void displayBid(Bid bid) {
    cout << bid.bidId << ": " << bid.title << " | " << bid.amount
    << " | " << bid.fund << endl;
    return;
}

void LinkedList::PrintList() {
    // FIXME (5): Implement print logic
    //dont loop with the head, loop with bid name, because you dont want head pointer to change
    Bid* bid = this->head; //start at list's beginning
    cout << "List Head: " << this->head << endl;
    while(bid!=nullptr){
        displayBid(*(bid));
        cout << "Printing Address: " << bid << endl;
        bid = bid->nextBid; //move to the next bid
    }
}

Bid getBid() {
    Bid bid;
    //enter bid title, amount, etc.    
    return bid;
}

int main(int argc, char* argv[]) {
    
    // process command line arguments
    string csvPath = "eBid_Monthly_Sales_Dec_2016.csv";
    
    LinkedList bidList;
    
    Bid bid;
    
    int choice = 0;
    while (choice != 9) {
        cout << "Menu:" << endl;
        cout << "  1. Enter a Bid" << endl;
        cout << "  2. Load Bids" << endl;

        switch (choice) {
            case 1:{
                Bid addBid;
                cout << "new Bid Object's address is " << &addBid << endl; //address of the pointer
                bidList.Append(addBid);
//                displayBid(bid);
                bidList.PrintList();
                break;
            }
                

Answer 1

append（）中的参数不应该是Bid，而应该是Bid*。这是因为，由于参数是通过函数中的值传递的，所以bid对象只是原始对象的副本
这意味着当分配

Answer 2

此处第行void LinkedList:：Append（Bid-Bid）Bid是一个局部变量，将在控件从函数返回后解除分配。

现在这里这个-

您可以做的是动态分配节点，并将其地址传递给Append方法。

void LinkedList::Append(Bid* bid) // function signature

添加节点：

Bid addBid;
bidList.Append(&addBid);

Answer 3

问题是您试图在append函数中为临时变量分配指针

void LinkedList::Append(Bid bid) { //<---I'm having trouble with this method
// FIXME (3): Implement append logic
if (this->head==nullptr){ //first node in a linked list
    cout << "initialize head and tail" << endl;
    this->head=&bid; //point to the bid
    this->tail=&bid;
}
else {
    cout << "Old Tail: " << this->tail << endl;
    this->tail->nextBid=&bid; //this refers to bid
    this->tail=&bid; //update last bid
    cout << "New Tail: " << &bid << endl;
    this->tail->nextBid=nullptr; //set pointer after last bid to null
}

将Bid对象传递给函数，而不是指针，然后将tail指针设置为指向该对象，但该对象将在函数结束后删除，因为它是在本地创建的。因此，tail将指向已删除的对象，这将导致未定义的行为（就我个人而言，我在Linux上遇到“分段错误”）。作为一个选项，您可以将指向Bid对象的指针传递给函数，所有操作都将正常工作，因为指针将被设置为在函数外部声明的有效Bid对象。

void LinkedList::Append(Bid* bid) { //<---I'm having trouble with this method
// FIXME (3): Implement append logic
if (this->head==nullptr){ //first node in a linked list
    cout << "initialize head and tail" << endl;
    this->head=bid; //point to the bid
    this->tail=bid;
}
else {
    cout << "Old Tail: " << this->tail << endl;
    this->tail->nextBid=bid; //this refers to bid
    this->tail=bid; //update last bid
    cout << "New Tail: " << bid << endl;
    this->tail->nextBid=nullptr; //set pointer after last bid to null
}