C++ Templates - LinkedList

EDIT -- Answered below, missed the angled braces. Thanks all.

I have been attempting to write a rudimentary singly linked list, which I can use in other programs. I wish it to be able to work with built-in and user defined types, meaning it must be templated.

Due to this my node must also be templated, as I do not know the information it is going to store. I have written a node class as follows -

template <class T> class Node
{
    T data; //the object information
    Node* next; //pointer to the next node element

public:
    //Methods omitted for brevity
};

My linked list class is implemented in a seperate class, and needs to instantiate a node when adding new nodes to the end of the list. I have implemented this as follows -

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

template <class T> class CustomLinkedList
{
    Node<T> *head, *tail;

public:

    CustomLinkedList()
    {
        head = NULL;
        tail = NULL;
    }

    ~CustomLinkedList()
    {

    }

    //Method adds info to the end of the list
    void add(T info)
    {
        if(head == NULL) //if our list is currently empty
        {
            head = new Node<T>; //Create new node of type T
            head->setData(info);
            tail = head;
        }
        else //if not empty add to the end and move the tail
        {
            Node* temp = new Node<T>;
            temp->setData(info);
            temp->setNextNull();
            tail->setNext(temp);
            tail = tail->getNext();
        }
    }

    //print method omitted
};

I have set up a driver/test class as follows -

#include "CustomLinkedList.h"
using namespace std;

int main()
{
    CustomLinkedList<int> firstList;

    firstList.add(32);
    firstList.printlist();
    //Pause the program until input is received
    int i;
    cin >>开发者_JS百科 i;

    return 0;
}

I get an error upon compilation however - error C2955: 'Node' : use of class template requires template argument list - which points me to the following line of code in my add method -

Node* temp = new Node<T>;

I do not understand why this has no information about the type, since it was passed to linked list when created in my driver class. What should I be doing to pass the type information to Node?

Should I create a private node struct instead of a seperate class, and combine the methods of both classes in one file? I'm not certain this would overcome the problem, but I think it might. I would rather have seperate classes if possible though.

Thanks, Andrew.

While the answers have already been provided, I think I'll add my grain of salt.

When designing templates class, it is a good idea not to repeat the template arguments just about everywhere, just in case you wish to (one day) change a particular detail. In general, this is done by using typedefs.

template <class T>
class Node
{
public:
  // bunch of types
  typedef T value_type;
  typedef T& reference_type;
  typedef T const& const_reference_type;
  typedef T* pointer_type;
  typedef T const* const_pointer_type;

  // From now on, T should never appear
private:
  value_type m_value;
  Node* m_next;
};


template <class T>
class List
{
  // private, no need to expose implementation
  typedef Node<T> node_type;

  // From now on, T should never appear
  typedef node_type* node_pointer;

public:
  typedef typename node_type::value_type value_type;
  typedef typename node_type::reference_type reference_type;
  typedef typename node_type::const_reference_type const_reference_type;
  // ...

  void add(value_type info);

private:
  node_pointer m_head, m_tail;
};

It is also better to define the methods outside of the class declaration, makes it is easier to read the interface.

template <class T>
void List<T>::add(value_type info)
{
  if(head == NULL) //if our list is currently empty
  {
    head = new node_type;
    head->setData(info);
    tail = head;
  }
  else //if not empty add to the end and move the tail
  {
    Node* temp = new node_type;
    temp->setData(info);
    temp->setNextNull();
    tail->setNext(temp);
    tail = tail->getNext();
  }
}

Now, a couple of remarks:

• it would be more user friendly if List<T>::add was returning an iterator to the newly added objects, like insert methods do in the STL (and you could rename it insert too)
• in the implementation of List<T>::add you assign memory to temp then perform a bunch of operations, if any throws, you have leaked memory
• the setNextNull call should not be necessary: the constructor of Node should initialize all the data member to meaningfull values, included m_next

So here is a revised version:

template <class T>
Node<T>::Node(value_type info): m_value(info), m_next(NULL) {}

template <class T>
typename List<T>::iterator insert(value_type info)
{
  if (m_head == NULL)
  {
    m_head = new node_type(info);
    m_tail = m_head;
    return iterator(m_tail);
  }
  else
  {
    m_tail.setNext(new node_type(info));
    node_pointer temp = m_tail;
    m_tail = temp.getNext();
    return iterator(temp);
  }
}

Note how the simple fact of using a proper constructor improves our exception safety: if ever anything throw during the constructor, new is required not to allocate any memory, thus nothing is leaked and we have not performed any operation yet. Our List<T>::insert method is now resilient.

Final question:

Usual insert methods of single linked lists insert at the beginning, because it's easier:

template <class T>
typename List<T>::iterator insert(value_type info)
{
  m_head = new node_type(info, m_head); // if this throws, m_head is left unmodified
  return iterator(m_head);
}

Are you sure you want to go with an insert at the end ? or did you do it this way because of the push_back method on traditional vectors and lists ?

Might wanna try

Node<T>* temp = new Node<T>;

Also, to get hints on how to design the list, you can of course look at std::list, although it can be a bit daunting at times.

You need:

Node<T> *temp = new Node<T>;

Might be worth a typedef NodeType = Node<T> in the CustomLinkedList class to prevent this problem from cropping up again.

That line should read

Node<T>* temp = new Node<T>;

Same for the next pointer in the Node class.

As said, the solution is

Node<T>* temp = new Node<T>;

... because Node itself is not a type, Node<T> is.

And you will need to specify the template parameter for the Node *temp in printlist also.

// file: main.cc

#include "linkedlist.h"

int main(int argc, char *argv[]) {
    LinkedList<int> list;
    for(int i = 1; i < 10; i++) list.add(i);
    list.print();
}

// file: node.h

#ifndef _NODE_H
#define _NODE_H

template<typename T> class LinkedList;
template<typename T>class Node {
    friend class LinkedList<T>;
    public:
        Node(T data = 0, Node<T> *next = 0)
            : data(data), next(next)
        { /* vacio */ }
    private:
        T data;
        Node<T> *next;
};

#endif//_NODE_H

// file: linkedlist.h

#ifndef _LINKEDLIST_H
#define _LINKEDLIST_H

#include <iostream>
using namespace std;

#include "node.h"

template<typename T> class LinkedList {
    public:
        LinkedList();
        ~LinkedList();
        void add(T);
        void print();
    private:
        Node<T> *head;
        Node<T> *tail;
};

#endif//_LINKEDLIST_H

template<typename T>LinkedList<T>::LinkedList()
    : head(0), tail(0)
{ /* empty */ }

template<typename T>LinkedList<T>::~LinkedList() {
    if(head) {
        Node<T> *p = head;
        Node<T> *q = 0;

        while(p) {
            q = p;
            p = p->next;
            delete q;
        }

        cout << endl;
    }
}

template<typename T>LinkedList<T>::void add(T info) {
    if(head) {
        tail->next = new Node<T>(info);
        tail = tail->next;
    } else {
        head = tail = new Node<T>(info);
    }
}

template<typename T>LinkedList<T>::void print() {
    if(head) {
        Node<T> *p = head;

        while(p) {
            cout << p->data << "-> ";
            p = p->next;
        }

        cout << endl;
    }
}

You Should add new node in this way

Node<T>* temp=new node<T>;

Hope you Solved :)

#include<iostream>
using namespace std;

template < class data > class node {
    private :
        data t;
        node<data > *ptr;
    public:
    node() {
        ptr = NULL;
    }
    data get_data() {
        return t;
    }
    void set_data(data d) {
        t = d;
    }
    void set_ptr(node<data > *p) {
        ptr = p;
    }
    node * get_ptr() {
        return ptr;
    }
};
template <class data > node < data > * add_at_last(data  d  , node<data > *start) {
    node< data > *temp , *p  = start;
    temp = new node<data>();
    temp->set_data(d);
    temp->set_ptr(NULL);
    if(!start) {
        start = temp;
        return temp;
    }
    else {
        while(p->get_ptr()) {
            p = p->get_ptr();
        }
        p->set_ptr(temp);
    }
}
template < class data > void display(node< data > *start) {
    node< data > *temp;
    temp = start;
    while(temp != NULL) {
        cout<<temp->get_data()<<" ";
        temp = temp->get_ptr();
    }
    cout<<endl;
}
template <class data > node < data > * reverse_list(node<data > * start) {
    node< data > *p = start , *q = NULL , *r = NULL;
    while(p->get_ptr()) {
        q = p;
        p = p->get_ptr();
        q->set_ptr(r);
        r = q;
    }
    p->set_ptr(r);
    return p;
}
int main() {
    node < int > *start;
    for(int i =0 ; i < 10 ; i ++) {
        if(!i) {
            start = add_at_last(i , start);
        }
        else {
            add_at_last(i , start);
        }
    }
    display(start);
    start = reverse_list(start);
    cout<<endl<<"reverse list is"<<endl<<endl;
    display(start);
}