Question

I’m trying to create a linked list for a project. I’ve got these two file (one .h and one .cpp). I’m not sure how to make a copy constructor so I’m not sure if that has anything to do with it. I suppose if someone wanted to point me in the right direction on that as well, it would be helpful. Thank you.

#include <iostream>
#include "studentList.h"

using namespace std;

// Default Constructor for StudentList
// Creates Dummy Head Node for a new empty list
StudentList::StudentList ()
{
// Create the dummy head node
Node* Head;             // Creates Head Node
Head = new Node;        
Head->next = NULL;      // Sets pointer to NULL by default
}

//Copy Constructor
StudentList::StudentList(const StudentList& list)
{

}

void StudentList::addStudentList(Student newStudent)
{

Getting the error here!!!!!!

if (Head->next == NULL)
{
    Head->next->student = newStudent;
    Head->next->prev = Head;
    Head->next->next = NULL;
}
}

Here is the .h file

#include <iostream>
#include "Student.h"

using namespace std;


class StudentList{

public:
//Default Constructor
StudentList();
//Copy Constructor
StudentList(const StudentList& list);
//Add Student Method
void addStudentList(Student);


private:
// Node struct to hold Student data and with pointers to a previous and next node in linked list
struct Node {
Student student;
Node* prev;
Node* next;
};

};

Answer 1

Head is supposed to be a member. The pointer Head you create has automatic storage. It goes out of scope when the constructor is finished, and you get a dangling reference.

class StudentList{

public:
//Default Constructor
  StudentList();
//Copy Constructor
  StudentList(const StudentList& list);
//Add Student Method
  void addStudentList(Student);


private:
// Node struct to hold Student data and with pointers to a previous and next node in linked list
  struct Node {
    Student student;
    Node* prev;
    Node* next;
  };

  Node* Head;

};


StudentList::StudentList ()
{
  Head = new Node;        
  Head->next = NULL;      // Sets pointer to NULL by default
}

On a side note, you should indent the code you put between curly braces. It makes it easier fo the human reader to group related elements together, an to spot code blocks.

As for the copy c’tor, as you can see, it takes a reference to an existing object, and constructs a new one out of it. The defaulut copy c’tor supplied by the compiler does a shallow copy. Meaning is a and b are lists, than a.Head and b.Head point to the same starting elements. You can overrride it to do a deep copy with something like this:

StudentList::StudentList(const StudentList& list)
{
  Node* Head = new Node;
  Node* tmp = Head;
  Node* iter = list.Head;
  while (iter)
  {
    *tmp = *iter;
    tmp->next = NULL;
    if (iter->next)
      tmp->next = new Node;
    tmp = tmp->next;
    iter = iter->next;
  }
}

I of course neglected tmp->prev but that’s the general idea for a signly linked list.