Printing a Single Node from a Doubly Linked List Class

Question

i'm attempting to implement three methods currently a get_first(), get_last() and print_node(). get_first() will return the head of a list, get_last() the tail, and print_node() will just print the data field of a node sent to it. im trying to implement but continually getting pointer errors for any changes that i make.

here's my node.h header:

class Node
{
    private:
        int data;
        Node *next;
        Node *prev;
        friend class LinkedList;
};

class LinkedList
{
    private:
        Node *head;
        Node *tail;

    public:
        LinkedList();
        ~LinkedList();

        bool empty();

        void insert_left(int v);
        void insert_right(int v);
        Node* get_first();
        Node* get_last();
        void print_list();
        void print_node(Node *n);
        void remove_left();
        void remove_right();


    protected:
        void add(Node *v, int d);
        void remove(Node *v);
};

here are the relevant portions of my list.cpp class implementation file:

#include <iostream>
#include "node.h"
using namespace std;

LinkedList :: LinkedList()
{
    head  = new Node;
    tail = new Node;

    head->next = tail;
    tail->prev = head;
}

LinkedList :: ~LinkedList()
{
    while(!empty())
    {
        remove_left();
    }
    delete head;
    delete tail;
}

void LinkedList :: add(Node *v, int d)
{
    Node *u = new Node;
    u->data = d;
    u->next = v;
    u->prev = v->prev;
    v->prev->next = v->prev = u;
}

void LinkedList :: print_list()
{
    Node *tmp = head;
    while(tmp != NULL)
    {
        cout << tmp->data << endl;
        tmp = tmp->next;
    }
}

void LinkedList :: print_node(Node *n)
{
    Node *tmp = n;
    cout << tmp->data << endl;
}

Node LinkedList :: get_first()
{
    return head;
}

Node LinkedList :: get_last()
{
    return tail;
}

finally here's my main function in a file called main.cpp:

#include <cstdlib>
#include <iostream>
#include "list.cpp"
using namespace std;

int main(int agrc, char  **argv)
{
    LinkedList *l = new LinkedList();
    //LinkedList *m = new LinkedList();

    l->insert_left(200);
    l->insert_left(700);
    l->insert_left(300);

    Node *temp = l->get_first();
    //l->print_list();

    l->print_node(temp);

    delete l;

    return 0;
}

here's the current error output:

g++ main.cpp -o main
In file included from main.cpp:3:
list.cpp:85: error: prototype for ‘Node LinkedList::get_first()’ does not match any in     class ‘LinkedList’
node.h:24: error: candidate is: Node* LinkedList::get_first()
list.cpp:90: error: prototype for ‘Node LinkedList::get_last()’ does not match any in     class ‘LinkedList’
node.h:25: error: candidate is: Node* LinkedList::get_last()
make: *** [main] Error 1

i'm not sure of the exact changes to make but i think it has to do with how i'm returning the head in the get_first() and last() functions. Please excuse the length of the post.

Answer 1

You are returning Node* in function declaration but in definition you have Node as the return type. Use this

Node* LinkedList :: get_first()
{
    return head;
}

Node* LinkedList :: get_last()
{
    return tail;
}

Answer 2

Data members head and tail are defined as

    Node *head;
    Node *tail;

that is they are pointers to Node. So if any function returns either head or tail then its return type has to be Node * So these member function definitions

Node LinkedList :: get_first()
{
    return head;
}

Node LinkedList :: get_last()
{
    return tail;
}

are wrong. They return head and tail but have no the return type Node * and their definitions do not coinside with theor declarations in the class.

Also the constructor definition is wrong. It shoild look as

LinkedList :: LinkedList() : head( nullptr ), tail( nullptr )
{
}

In this case member function empty should be declared as

bool empty() const;

and defined as

bool empty() const { return ( head == nullptr ); }