Exception thrown: read access violation (Linked List)

I'm making a singly Linked List called Tracks, why is it throwing exception when I try to make a ptr of type Tracks and assign head to it. It also gives same error when I assign mbidd value to node->trackRef->mbid

template <typename T1>  class Tracks  {  public:      //data members      T1 trackNumber;      MMD<T1,string, float>* trackRef;        //connections      Tracks* next;      Tracks* head;        Tracks()      {          next = nullptr;          head = nullptr;                   trackNumber = 0;          trackRef->mbid = 0;                 //mbid is data member of MMD      }        Tracks* createList(T1 tN, T1 mbidd)      {          Tracks<T1>* node= new Tracks<T1>();          Tracks<T1>* ptr = new Tracks<T1>();           ptr = head;                           //gives error          node->trackNumber = tN;          node->trackRef->mbid = mbidd;         //gives error            if (head == nullptr)          {              head = node;              node->next = nullptr;          }          else          {              while (ptr->next != nullptr)                  ptr = head->next;              ptr->next = node;          }          return head;        }  };  

Please check the error

https://stackoverflow.com/questions/67327613/exception-thrown-read-access-violation-linked-list April 30, 2021 at 10:53AM