Reputation: 1169

Accessing an element within my custom linked list class

I'm building my own linked list class and I'm having some issues figuring out how to write some functions to help me traverse this list. This is my first time building a linked list from scratch, so if my approach is unconventional please let me know what might be more conventional.

I'd like write a function, within the List class that allows me to increment to the next element called getNext() as well as one that getPrev();

I wrote getNext like this:

T* getNext(){return next;}

However it tells me next is not declared within the scope. I'd also like to write a function that lets me access and modify the object within the list. I was considering using the bracket operator, but first I need to write a function to return the data member. Perhaps If I take a similar approach as I did within my pop functions.. thinking about it now. However, I'd still appreciate any advice.

Here is my List class:

#ifndef LIST_H
#define LIST_H


//List Class
template <class T>
class List{
    struct Node {
        T data;
        Node *next;
        Node *prev;
        //Constructs Node Element
        Node(T t, Node* p, Node* n) { data = (t); prev = (p); next = (n); }
//      T *getNext() {return next;}
    };
        Node *head;
        Node *tail;
public: 
    //Constructor
    List() { head = NULL; tail=NULL; }
    //Destructor
    ~List() { 
        while(head){
            Node * temp(head);
            head = head->next;
            delete temp;
        }
    }
    //is empty
    bool empty() const {return (!head || !tail ); }
    operator bool() const {return !empty(); }
    //Push back
    void push_back(T data) {
        tail = new Node(data, tail, NULL);
        if(tail->prev) //if the node in front of tail is initilized
            tail->prev->next = tail;

        if( empty() )
            head = tail;
    }
    //Push front
    void push_front(T data) {
        head = new Node(data, NULL, head);
        if(head->next)//if the node following head is initilized
            head->next->prev = head;

        if( empty() )
         tail = head;
    };
    T pop_back() {
        if( empty() )
            throw("Error in List: List is empty\n");

        Node* temp(tail);
        T data(tail->data);
        tail = tail->prev;

        if( tail )
            tail->next = NULL;
        else
            head = NULL;

        delete temp;
        return data;
    }
    T pop_front() {
        if (empty())
            throw("Error in List: List is empty\n");

        Node* temp(head);
        T data(head->data);
        head = head->next;

        if(head)
            head->prev=NULL;
        else
            tail = NULL;

        delete temp;
        return data;
    }

    T getNext(){return next;}
};


#endif

Upvotes: 1

Answers (3)

pippin1289

Reputation: 4939

getNext should be part of the struct Node and return a Node*

Node* getNext() { return next; }

Then from that you can get the value.

If you have to have it part of the list itself, which I would not recommend it will need to take a parameter of what Node you would like the next of:

Node* getNext(Node* n) {return n->next;}

Again, I recommend the first option.

Here is an approximate whole class with both of these:

template<typename T>
class List {
  public:
    struct Node {
      Node* next, prev;
      T data;

      //some constructor and stuff

      Node* Next() {return next;}
    }

    //some constructors and other functions

    Node* getNext(Node* _n) {return _n->Next();}
}

then to use:

int main() {
  List<int> l;
  //add some stuff to the list
  //get the head of the list
  List<int>::Node* head = l.head; //or some corresponding function

  //then
  List<int>::Node* next = head->Next();
  //or
  List<int>::Node* next2 = l.getNext(head);
}

Upvotes: 1