class variable getting altered by code that were seemingly not executed - c++

Question

So I'm doing this singly linked list implementation for an school assignment. A struct of "evaluation" was defined in the header file and a object of "mylinkedlist" would hold the pointer to the head of the linked list. This should be an easy project but for some reason, whenever I try to call the add function, the head pointer was altered despite a) not being called and b) the head pointer was seemingly altered before the add function was even called
below is a minimum repro of the problem
//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;
}
See how the head was changed between "list head is 2nd" and "list head is 3rd" despite the head not being called. And another strange thing was that if i remove the add function or the call to the add function , the problem would disappear despite the call to the add function coming after the couts.
And inside the add method, the if statement seemed to be the problem once again despite being coming after the cout line and not executing (the b==0 outputting false). The only way i knew that it has something to do with the persisting problem is that if i delete it the problem went away.

Answer 1

The same node assigned to tempt is repeatedly added, so modifying the node will affect other references of the node.
It seems you want to allocate new nodes for each input. To do that, allocate new elements inside the loop.
// 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);