Deep copy of a list?

[Kman]

I need to perform a deep copy of a list, but I'm not sure how to continue. Here's my struct declaration, which is set to private:

  struct BookListNode
		{
			Book bookData;
			BookListNode *next;
		};

And here's the function, otherList being the variable I have to make a deep copy of:

BookList::BookList(const BookList& otherList)
{
BookListNode *current = otherList;  

	BookListNode *copy = new BookListNode;
	copy->next = NULL;

	while (current != NULL) {
		(copy->bookData) = (current->bookData);
		*(copy->next) = *(current->next);

		copy = copy->next;
		current = current->next;
	}
	return copy;

[AbstractionAnon]

Line 12: Problem here is you now have two lists pointing to the the same BookListNode.
You're going to have a problem, when you try to delete a node from your BookList. Deleting an entry from a BookList should delete the associated BookListNode. Once you do that, the other BookList is going to point to a BookListNode that no longer exists.

You should either use smart pointers, or create a copy of the BookListNode.

[Kman]

Like this?

BookListNode *current = otherList;  

	BookListNode *copy = new BookListNode;
	BookListNode *secondCopy = new BookListNode;

	copy->next = NULL;

	//traverses the list
	while (current != NULL) {
		(copy->bookData) = (current->bookData);
		*(copy->next) = *(current->next);

		secondCopy = copy->next;
		current = current->next;
	}
	return secondCopy;

[Kman]

But now it says that for otherList, "no suitable conversion function from 'const BookList' to 'BookList::BookListNode* exists", which I understand how it's incorrect, but I'm not sure how else to work otherList into the code.

[keskiverto]

How does class BookList object point to (the first) BookListNode?


Why doesn't the BookListNode have a custom constructor?

[Kman]

The BookListNode struct is declared as a private member of the BookList class. I'm supposed to code this without giving BookListNode its own constructor.

[keskiverto]

The BookListNode struct is declared as a private member of the BookList class.

struct BookListNode is a type.
class BookList is a type. BookList has member variables and member methods.

What is the name and exact type of the member variable of BookList that is or points to an instance of BookListNode?

[Kman]

I believe it's "first."

class BookList
{
	private:
		struct BookListNode
		{
			Book bookData;
			BookListNode *next;
		};

		int length;

		//Create a sample first item of the list
		BookListNode *first;

       public: .....

[keskiverto]

How we are talking. That line 12 comment, however, is way off.
If every List has at least one node, what is the value hold in that node?

A list can be empty, with no nodes at all. The first is a pointer, not a node. The pointer on empty list should have value nullptr.

BookList::BookList( const BookList& otherList ) {
  BookListNode * other = otherList.first;
  BookListNode * * tail = &first;

Does your BookList have a "append to tail" method? If yes, show its code.


The last shown line in your constructors has had a return statement. Constructors don't "return" a value.

[Kman]

It will have an append method, but I haven't coded it yet, and I already deleted the return statement. Could you explain what those double asterisks do exactly?

[keskiverto]

Variable declarations are best to read from right to left. Those say:

Name 'other' is a pointer variable to type BookListNode object(s).
Name 'tail' is a pointer variable to a pointer to type BookListNode object(s).

[Kman]

Got it, thanks for all the help! One more thing, this is what I have for my Add method (which is separate from the "append to tail" method). This is supposed to add a new Book to the list (Book being another class I imported). This isn't correct, but am I in the ballpark?

void BookList::Add(const Book e)
{
	struct BookListNode newNode;

	for (int i = 0; i < length; i++)
	{
		newNote.bookData->next = e;
	}

	length++; //I set the length declaration to be equal to 0.
};

[keskiverto]

Where in the list do you want to add? To begin, end, or somewhere in the middle (to keep list sorted)?

What is 'length'?

[Kman]

It doesn't matter where I add it, so the end would be fine. Length is just an ongoing count of how many entries are in the list.