Need some advice with classes in C++, doubly linked list - c++

Question

I'm trying to print all the elements of the list, but I'm getting a wrong output.
The code gets 2, 0 and 10, and when I call the procedure "travel_in" it only shows 0, 2.
And have some doubts with my del_start(), it deletes the 0 and not the 2..
What I'm doing wrong?
Compiled in Windows 64bits with Cygwin
Output
2 0 10 0 2
Here is the code
# include < iostream >
# include < stdio.h >
using namespace std;
template <class clali>
class double_list
{
protected:
clali node1;
clali *listad;
public:
double_list()//constructor
{
listad=NULL;
}
void insert_strt(clali node1)
{
clali *temp;
temp=new clali;
*temp=node1;
//check if list is not empty
if (listad==NULL)
{
listad=temp;
listad->next=NULL;
listad->before=NULL;
}
else
{
temp->next=listad;
listad->before=temp;
temp->before=NULL;
listad=temp;
}
}
int vertam()
{
int res=0;
clali *temp;
temp=listad;
if (temp==NULL)
{
cout<<"Empty list!"<<endl;
res=0;
}
else
while(temp!=NULL)
{
res++;
temp=temp->next;
}
return res;
}
void insert_mid(clali node1, int pos)
{
int i;
clali *temp,*temp2;
temp2=new clali;
temp=listad;
if(pos<vertam)
{
for(i=1;i<pos;i++)
temp=temp->next;
*temp2=node1;
temp2->next=temp->next;
temp->before=listad;
temp->next=temp2;
}
else
cout<<"Cant show the data!"<<endl;
}
clali del_start()
{
clali a,*temp;
a=*listad;
temp=listad;
listad=listad->next;
delete temp;
return a;
}
void insert_end(clali node1)
{
clali *temp,*temp2;
temp=listad;
while(temp->next!=NULL)
{
temp=temp->next;
}
temp2=new clali;
*temp2=node1;
temp->next=temp2;
temp2->before=temp;
temp2->next=NULL;
}
clali clear_end()
{
clali b,*temp,*temp2;
int j=1;
temp=listad;
do
{
temp=temp->next;
cout<<"Element : "<<j<<endl;
j++;
}while(temp->next!=NULL);
b=*temp;
temp2=temp->before;
temp2->next=NULL;
// delete temp;
return b;
}
void travel_in()
{
clali *temp;
temp=listad;
while(temp->next!=NULL)
{
cout<<temp->data<<endl;
temp=temp->next;
}
}
};
struct integer
{
int data;
integer*next,*before;
};
typedef struct integer Integer;
int main()
{
Integer node;
node.next=NULL;
node.before=NULL;
double_list<Integer> test_list;
test_list.insert_strt(node);
node.data=2;
cout<<node.data<<endl;
test_list.insert_end(node);
node.data=0;
cout<<node.data<<endl;
test_list.insert_end(node);
node.data=10;
cout<<node.data<<endl;
test_list.del_start();
test_list.travel_in();
}

Answer 1

I see at least one obvious bug. Initial analysis indicates that the listad class member is the pointer to the first element in the doubly-linked list. In that case, the following is obviously wrong (reformated for legibility, please indent your code correctly):
void insert_mid(clali node1, int pos)
{
int i;
clali *temp,*temp2;
temp2=new clali;
The purpose of this class method is, apparently, to insert the new node in the middle of the linked list.
temp2 is the new node.
temp=listad;
if(pos<vertam)
{
for(i=1;i<pos;i++)
temp=temp->next;
temp appears to be the insert position in the middle of the list.
temp->before=listad;
For some unclear reason this code attempts to set the before pointer of an existing node in the middle of the list to the head of the list. This makes no sense, and is wrong.

Answer 2

Let's go step by step in main().
When you first call insert_strt() the argument node has garbage value for member data. So an Integer object with some garbage value for data gets inserted at the start of test_list. Then you insert Integer objects with data 2 and 0, respectively, at the end of the list.
Later, you delete the first clali object from test_list which deletes the object with garbage value in its data field. So, after deletion, you have objects with data value 2, and 0 in the list in that order.
At the end, you print the list with travel_in() but it does not do what you think it does. What it is actually doing is that if the list has at least one element then it prints all but the last element in the list. If the list is empty, it will cause a segmentation fault (in the condition of while loop as temp would be NULL). So it will print: 2 (but your list has 2 and 0).
You can write travel_in() as follows.
void travel_in()
{
clali *temp = listad;
while(temp != NULL)
{
cout << temp->data << " ";
temp = temp->next;
}
cout << endl;
}
By the way, comment/remove the cout statements in the main() function. They may confuse you.