Doubly linked list question

[nearc]

Can someone explain this part to me?
1.
double_llist()
{
start = NULL;
}
what does it do? why it doesn't start with void?
2.
double_llist dl;
What does this do in main? what is d1?
3.
What does pointer start do in this code? is it head?

Thank you for answers

  /*
 * C++ Program to Implement Doubly Linked List 
 */
#include<iostream>
#include<cstdio>
#include<cstdlib>
/*
 * Node Declaration
 */
using namespace std;
struct node
{
    int info;
    struct node *next;
    struct node *prev;
}*start;
 
/*
 Class Declaration 
 */
class double_llist
{
    public:
        void create_list(int value);
        void add_begin(int value);
        void add_after(int value, int position);
        void delete_element(int value);
        void search_element(int value);
        void display_dlist();
        void count();
        void reverse();
        double_llist()
        {
            start = NULL;  
        }
};
 
/*
 * Main: Conatins Menu
 */
int main()
{
    int choice, element, position;
    double_llist dl;
    while (1)
    {
        cout<<endl<<"----------------------------"<<endl;
        cout<<endl<<"Operations on Doubly linked list"<<endl;
        cout<<endl<<"----------------------------"<<endl;         
        cout<<"1.Create Node"<<endl;
        cout<<"2.Add at begining"<<endl;
        cout<<"3.Add after position"<<endl;
        cout<<"4.Delete"<<endl;
        cout<<"5.Display"<<endl;
        cout<<"6.Count"<<endl;
        cout<<"7.Reverse"<<endl;
        cout<<"8.Quit"<<endl;
        cout<<"Enter your choice : ";
        cin>>choice;
        switch ( choice )
        {
        case 1:
            cout<<"Enter the element: ";
            cin>>element;
            dl.create_list(element);
            cout<<endl;
            break;
        case 2:
            cout<<"Enter the element: ";
            cin>>element;
            dl.add_begin(element);
            cout<<endl;
            break;
        case 3:
            cout<<"Enter the element: ";
            cin>>element;
            cout<<"Insert Element after postion: ";
            cin>>position;
            dl.add_after(element, position);
            cout<<endl;
            break;
        case 4:
            if (start == NULL)
            {                      
                cout<<"List empty,nothing to delete"<<endl;   
                break;
            }
            cout<<"Enter the element for deletion: ";
            cin>>element;
            dl.delete_element(element);
            cout<<endl;
            break;
        case 5:
            dl.display_dlist();
            cout<<endl;
            break;
        case 6:
            dl.count();
            break;    
        case 7:
            if (start == NULL)
            {
                cout<<"List empty,nothing to reverse"<<endl;
                break;
            }
            dl.reverse();
            cout<<endl;
            break;
        case 8:
            exit(1);
        default:
            cout<<"Wrong choice"<<endl;
        }
    }
    return 0;
}
 
/*
 * Create Double Link List
 */
void double_llist::create_list(int value)
{
    struct node *s, *temp;
    temp = new(struct node); 
    temp->info = value;
    temp->next = NULL;
    if (start == NULL)
    {
        temp->prev = NULL;
        start = temp;
    }
    else
    {
        s = start;
        while (s->next != NULL)
            s = s->next;
        s->next = temp;
        temp->prev = s;
    }
}
 
/*
 * Insertion at the beginning
 */
void double_llist::add_begin(int value)
{
    if (start == NULL)
    {
        cout<<"First Create the list."<<endl;
        return;
    }
    struct node *temp;
    temp = new(struct node);
    temp->prev = NULL;
    temp->info = value;
    temp->next = start;
    start->prev = temp;
    start = temp;
    cout<<"Element Inserted"<<endl;
}
 
/*
 * Insertion of element at a particular position
 */
void double_llist::add_after(int value, int pos)
{
    if (start == NULL)
    {
        cout<<"First Create the list."<<endl;
        return;
    }
    struct node *tmp, *q;
    int i;
    q = start;
    for (i = 0;i < pos - 1;i++)
    {
        q = q->next;
        if (q == NULL)
        {
            cout<<"There are less than ";
            cout<<pos<<" elements."<<endl;
            return;
        }
    }
    tmp = new(struct node);
    tmp->info = value;
    if (q->next == NULL)
    {
        q->next = tmp;
        tmp->next = NULL;
        tmp->prev = q;      
    }
    else
    {
        tmp->next = q->next;
        tmp->next->prev = tmp;
        q->next = tmp;
        tmp->prev = q;
    }
    cout<<"Element Inserted"<<endl;
}
 
/*
 * Deletion of element from the list
 */
void double_llist::delete_element(int value)
{
    struct node *tmp, *q;
     /*first element deletion*/
    if (start->info == value)
    {
        tmp = start;
        start = start->next;  
        start->prev = NULL;
        cout<<"Element Deleted"<<endl;
        free(tmp);
        return;
    }
    q = start;
    while (q->next->next != NULL)
    {   
        /*Element deleted in between*/
        if (q->next->info == value)  
        {
            tmp = q->next;
            q->next = tmp->next;
            tmp->next->prev = q;
            cout<<"Element Deleted"<<endl;
            free(tmp);
            return;
        }
        q = q->next;
    }
     /*last element deleted*/
    if (q->next->info == value)    
    { 	
        tmp = q->next;
        free(tmp);
        q->next = NULL;
        cout<<"Element Deleted"<<endl;
        return;
    }
    cout<<"Element "<<value<<" not found"<<endl;
}
 
/*
 * Display elements of Doubly Link List
 */
void double_llist::display_dlist()
{
    struct node *q;
    if (start == NULL)
    {
        cout<<"List empty,nothing to display"<<endl;
        return;
    }
    q = start;
    cout<<"The Doubly Link List is :"<<endl;
    while (q != NULL)
    {
        cout<<q->info<<" <-> ";
        q = q->next;
    }
    cout<<"NULL"<<endl;
}
 
/*
 * Number of elements in Doubly Link List
 */
void double_llist::count()
{ 	
    struct node *q = start;
    int cnt = 0;
    while (q != NULL)
    {
        q = q->next;
        cnt++;
    }
    cout<<"Number of elements are: "<<cnt<<endl;
}
 
/*
 * Reverse Doubly Link List
 */
void double_llist::reverse()
{
    struct node *p1, *p2;
    p1 = start;
    p2 = p1->next;
    p1->next = NULL;
    p1->prev = p2;
    while (p2 != NULL)
    {
        p2->prev = p2->next;
        p2->next = p1;
        p1 = p2;
        p2 = p2->prev; 
    }
    start = p1;
    cout<<"List Reversed"<<endl; 
}

[jonnin]

1) this is the class constructor.
think,
int x = 3;
this creates a new integer, and assigns it a value of 3. If int were a class, it would be calling a constructor to do that.
It does not have a type, because c++ syntax. Google "why constructor has no return type in c++" for in-depth reasons, or just know it is a feature of the language. This constructor for your list class (or, apparently, someone else's list class) sets the start pointer to null.

2) you are creating a variable of the class type, much like my int x above, here dl is analogous to x, dl is a variable that you can use. the class itself cannot be used; it is a type, just as you cannot use int standalone as an integer but you make a variable of that type.

3) start appears to be 'head', yes.

[mudaliaraditya]

1)
the start is NULL because all variables are created with garbage values at start so we are initia;izing
(NULL means pointing nowhere)
void is a datatype,its not a value.
NULL is #define NULL 0,that means it points to nothing
2)
in order to use functions of a class unless they are a
static function of the class we cannot call a function
without the object hence we are creating an object of double_llist

3)

yes start is the head

[nearc]

I have 1 more question what does 136 line do???
If it's second time creating list it become NULL?

[dhayden]

The loop at lines 135 and 136 moves s down until it points to the last item in the list. This is a little odd since you'd expect a method called create_list() to make a new list. This create_list() method really adds the value to the end of the list. It would be more appropriately named add_end() to match the add_begin() method.

It's also odd that start is a global variable and not a member of the list. That means the class can only support a single list. Even when there are multiple instances of the class, they all refer to the same list because they all use the same start pointer.