Linked Lists in another Linked list - c++

Question

I'm trying to make a List that contains another linked lists and then I want to concatenate them into one list. I have some difficulties with this. So first I make this:
typedef List DList; - It makes a type of linked lists;
Then for example I make DList A and B. I insert some numbers like 1,2,3... in both of them;
Then I add them in the main linked list which is like:
List<DList> L;
L.Add_Node(A);
L.Add_Node(B);
And everything seems to be fine at this stage of the program. After this I'm trying to make a function to concatenate the separete Linked lists. I make a new List NewList where the values will be stored. And here comes my problem. I don't know how to write the function.
I make something like this: First I make a function that will loop the nodes in the L list(there are the two little lists). Then I go to my concatenation function. You can see them in the code below:
template<class T>
void Start(List<T> L, List<T> NewList)
{
List<T> *temp = L.start_ptr; //I get the position of the first element;
while(temp)
{
NewList = Concat(NewList, temp->data);
temp = temp->next;
}
}
template<class T>
List Concat(List<T> L1, List<T> L2)
{
List<T> *temp1, *temp2;
List<T> NewList;
temp1 = L1.start_ptr;
temp2 = L2.start_ptr;
while(temp1)
{
NewList.Add_Node(temp1->data);
temp1 = temp1->next;
}
while(temp2)
{
NewList.Add_Node(temp2->data);
temp2 = temp2->next;
}
return NewList;
}
These are my functions. Maybe they are completely wrong... The compiler gives me error that the members start_ptr, data and next are private(And it's true). It gives me much more errors.. Something like this is written in the book from which I'm learning, but it is so complicated there... If you don't want to spend you time in helping me I will be thankful even if you give me a good source from which I can read more about this type of Lists.

Answer 1

In your function start, you are going through two List of type T.
But then, you're concatenating a whole list with a value of the other list, while you're concatenating two lists in your Concat method.
Also, you are passing the parameters by copy, which probably not something you want to do.
I think what you meant to do is something like that :
void Start(List< List <T> > & L, List<T> &NewList)
Passing by copy means that your program send a copy of the parameter to your function.
It means that the list you pass will be copied (which can be pretty heavy, if your list is big), and that only the copy will be modified.
On the new version of the Start function that I gave you, you can notice that I added &. It means that the value is passed by reference. Basically, it means that it is actually your object being used, and not a copy.
Finally, about the members being private, you should add public accessors, so that they can be used outside of you class.
In your case, you can add a public method in your class :
template<class T>
const List<T> *get_start_ptr() const
{
return (start_ptr);
}
And then add public methods to move, like :
template<class T>
const List<T> *next() const
{
return (next);
}
The accessor returns the value as const, to be sure that they are not involuntary modified during the process. Even if you obviously don't modify it in your code, it is a good practice to use constness as much as possible, and to be explicit when you actually want to modify a value.