When searching through a singly linked list, I get a nullptr error?

Question

C++ / C#

There is a singly linked list and when comparing elements, an error occurs when pointing to NULL . The problem arises when the CompAndFind
function is executed, I need to go through the list and find the appropriate elements, it seems to me that I need to loop the pointers. But I don't think this is the best solution.

#include "stdafx.h"
#include <iostream>
#include <stdlib.h> 
#include <cstring>

#include "Header1.h"

using namespace std;

struct OLS
{
  int RoomsCount;
  int floor;
  float square;

  struct OLS *ptr;
};

struct OLS *init(int RoomsCount, int floor, float square)
{
  struct OLS *lst;
  //int RoomsCount, int floor, float square, char*adress
  // выделение памяти под корень списка
  lst = (struct OLS*)malloc(sizeof(struct OLS));
  lst->RoomsCount = RoomsCount;
  lst->floor = floor;
  lst->square = square;
  lst->ptr = NULL; // это последний узел списка
  return(lst);

}

struct OLS * addelem(OLS *lst, int RoomsCount, int floor, float square)
{
  struct OLS *temp, *p;
  temp = (struct OLS*)malloc(sizeof(struct OLS));
  p = lst->ptr; // сохранение указателя на следующий узел
  lst->ptr = temp; // предыдущий узел указывает на создаваемый

  temp->RoomsCount = RoomsCount;// сохранение поля данных добавляемого узла
  temp->floor = floor;
  temp->square = square;

  temp->ptr = p; // созданный узел указывает на следующий элемент
  
  

  return(temp);
}
void show(OLS*str)
{
        cout << "Rooms count: ";
    cout << str->RoomsCount << endl;
    cout << "Floor:";
    cout << str->floor << endl;
    cout << "Square:";
    cout << str->square << endl;

}
void CompAndFind(OLS*str) 
{
  struct OLS *p;
  p = str;
  do {
    
    if(p->RoomsCount==p->ptr->RoomsCount)  
    {
         if(p->floor == p->ptr->floor) {
             if (p->square != p->ptr->square)
             {
             show(p);
             }
             }
    }


    p = p->ptr; // переход к следующему узлу
  } while (p != NULL);

}


 

int main()
{
  //OLS StructArr[2];
  OLS*first; OLS*second; OLS*third;
  first = init(4, 4, 2);
  second = addelem(first, 4, 5, 4);
  third = addelem(second, 2, 4, 30);
  
  //listprint(first);// first because it is root of all list
  CompAndFind(first);
    

   
  system("pause");
  
  return 0;

}

Answer 1

if(p->RoomsCount==p->ptr->RoomsCount)  // ptr == nullptr;

and cycle

spoiler

struct OLS
{
  int RoomsCount;
  int floor;
  float square;
  OLS *ptr;
  friend bool operator==(OLS& a, OLS& b);
};

typedef OLS* pOLS;

bool operator==(OLS& a, OLS& b)
{
    return (a.RoomsCount == b.RoomsCount) &&
           (a.floor == b.floor) &&
           (a.square != b.square);
}

void CompAndFind(pOLS& str) 
{
  if(!str) return;// nullptr
  pOLS p = str;
  while(p->ptr)
  {
      if(*p == *(p->ptr))
      {
          show(p);
      }
      p = p->ptr;
  }
}

Answer 2

The moment you take p->ptr->RoomsCount from the last element in the list, you have p-ptr == NULL.