How do you use access the information from a string*? - c++

Question

I am using a doubly linked list and I am trying to use the data that is passed by reference to insert a node before said data. I've used string* data = new string(s); to allocate memory however, when I try to use data I get an error.
#ifndef __DOUBLYLINKEDLIST_H__
#define __DOUBLYLINKEDLIST_H__
//
//
#include
#include
using namespace std;
class DoublyLinkedList {
public:
DoublyLinkedList();
~DoublyLinkedList();
void append (const string& s);
void insertBefore (const string& s);
void insertAfter (const string& s);
void remove (const string& s);
bool empty();
void begin();
void end();
bool next();
bool prev();
bool find(const string& s);
const std::string& getData() const;
private:
class Node
{
public:
Node();
Node(const string& data);
~Node();
Node* next;
Node* prev;
string* data;
};
Node* head;
Node* tail;
Node* current;
};
void DoublyLinkedList::insertBefore(const string& s)
{
Node* ptr = head;
string* data = new string(s);
if (head == NULL)
{
append(s);
return;
}
if (head == current)
{
//this is where I get an error...
this->data= new Node();
current->prev = head;
current = head;
return;
}

Answer 1

There is no reason to use a pointer to a string, which forces you to manage memory. Use a simple string instead.
But this is not the problem here. Here a local variable has the same name than a class member of a Node , and the member in node gets never initalized. Furtherthermore the DoublyLinkedList has itself no such member, so this->data is unknown. See my comments here:
void DoublyLinkedList::insertBefore(const string& s)
{
...
string* data = new string(s); // --> ok, but this is local variable
if (head == NULL)
{
append(s);
return; // ouch !!! memory leak !! data pointer is never freed
}
if (head == current)
{
//this is where I get an error...
this->data= new Node(); // --> 'this' is a DoublyLinkedList, not a Node
...
return;
}
Now this being said, is it possbile that you make a confusion between the DoublyLinkedList and the nodes it contains ? See here a start of correction, but you need to do more to handle the linking between the nodes:
void DoublyLinkedList::insertBefore(const string& s)
{
Node* ptr = head;
if (head == NULL)
{
append(s);
return;
}
if (head == current)
{
string* data = new string(s);
Node nd = new Node();
nd->data = data; // initialize node's pointer
nd->prev = ... // you need to link new node to the rest
nd->next = ...
... // and you need to update the previous and next node
return;
}
Now, as said in the first place, replace the pointer to string with a string. At least, you'll avoid leaking memory, shallow copies, and lots of other troubles. Then you can focus better on the real problems of a linked list data structure.