class 变量被看似未执行的代码更改

Question

所以我正在为一项学校作业执行此单链表实现。头文件中定义了一个“evaluation”结构，“mylinkedlist”对象将保存指向链表头部的指针。这应该是一个简单的项目，但出于某种原因，每当我尝试调用 add 函数时，尽管 a) 未被调用且 b) head 指针在调用 add 函数之前似乎已被更改

下面是问题的最小重现

//header file
#pragma once

#ifndef MYLINKEDLIST_H
#define MYLINKEDLIST_H

#include <iostream>
#include <process.h>
using namespace std;
const int maxSize = 20; // size string
struct Evaluation
{
    char student[maxSize] = { 'a', 'b','c','d','e','f','g','h','i','j','k','l','m','n','o','p','q','r','s','t' };
    int grade;
    Evaluation *next;
};

class myLinkedList {
private:
    Evaluation *head;
public:
    myLinkedList(Evaluation *);
    Evaluation *add(Evaluation *, int &);
    Evaluation * returnHead();

};

#endif#pragma once

//mylinkedlist.cpp
#include "pch.h"
#include "myLinkedList.h"
#include <iostream>
#include <string>

myLinkedList::myLinkedList(Evaluation *h) {
    this->head = h;
}

Evaluation * myLinkedList::returnHead() {
    return this->head;
}

Evaluation * myLinkedList::add(Evaluation *c, int &b) {//a is the first element 
    Evaluation *pointer = this->head;
    cout << "head in the beginning " << this->head->grade << endl;
    cout << "pointer in the beginning " << pointer->grade << endl;

    bool y = b == 0;
    cout << "b==0 " << y << endl;

    if (b == 0) {
        pointer = this->head;
        this->head = c;
        this->head->next = pointer;
        cout << "head after if " << this->head->grade << endl;
    }
    return this->head;
}

//main
#include "pch.h"
#include "myLinkedList.h"
#include <iostream>
#include <string>

int main() {
    Evaluation *first = new Evaluation();
    int choice;
    int grade = 0;
    int number = 0;
    first->grade = 20;
    myLinkedList *list = new myLinkedList(first);
    cout << "list head is " << list->returnHead()->grade << endl;
    Evaluation *tempt = new Evaluation();
    do
    {
        cout << "please enter the grade of student : ";
        cin >> grade;
        cout << "list head is 2nd " << list->returnHead()->grade << endl;
        tempt->grade = grade;
        cout << "list head is 3rd " << list->returnHead()->grade << endl;
        list->add(tempt, number); // added element, index
        number++;
    } while (true);
    return 0;
}

尽管头没有被调用，但看看头是如何在“列表头是第 2 个”和“列表头是第 3 个”之间变化的。另一件奇怪的事情是，如果我删除添加函数或对添加函数的调用，尽管在 couts 之后调用添加函数，问题也会消失。

在 add 方法中，if 语句似乎再次成为问题，尽管它位于 cout 行之后并且没有执行（b==0 输出 false）。我知道它与持续存在的问题有关的唯一方法是，如果我删除它，问题就会消失。

Answer 1

分配给tempt的同一个节点被重复添加，所以修改该节点会影响该节点的其他引用。

您似乎想为每个输入分配新节点。为此，在循环内分配新元素。

    // remove this
    //Evaluation *tempt = new Evaluation();
    do
    {
        // and move here (inside the loop)
        Evaluation *tempt = new Evaluation();
        cout << "please enter the grade of student : ";
        cin >> grade;
        cout << "list head is 2nd " << list->returnHead()->grade << endl;
        tempt->grade = grade;
        cout << "list head is 3rd " << list->returnHead()->grade << endl;
        list->add(tempt, number); // added element, index
        number++;
    } while (true);

class 变量被看似未执行的代码更改

class variable getting altered by code that were seemingly not executed

c++

struct

pointers

linked-list

singly-linked-list