C++ Queue from Linked List

Question

Good evening. I have been trying to implement a Queue class in C++, taking a previously created Linked List class as a base. Linked Linked List:

#include <cstddef>
#include <iostream>
#include <cstdio>

using namespace std;

template <class T>
class LinkedList {

public:
    LinkedList() {
        head = NULL;
    }
    ~LinkedList() {
        MakeEmpty();
    }

    struct Node {
        T value;
        Node *next;
    };

    Node* getHead() {
        return head;
    }

    void Print();
    void Insert();
    void MakeEmpty();

private:
    Node *head;     // Head of the linked list.
};

Queue class:

#include "LinkedList.h"

template <class T>
class Queue {

public:
    Queue() {
        LinkedList<T>::Node *tnode = Q.getHead();
    }

    ~Queue() {
        Q.MakeEmpty();
    }

    void Enqueue( T x ) {
        LinkedList<T>::Node *cnode = Q.getHead();

        //Find the last element of Q
        while( cnode -> next != NULL ) {
            cnode = cnode -> next;
        }
        //Add x to the end of the queue
        Q.Insert( x );
    }

    void Dequeue() {
        LinkedList<T>::Node *hnode = Q.getHead();
        //Rest of function
    }

    void Print() {
        Q.PrintList();
    }

private:
    LinkedList<T> Q;
};

As you probably noticed, I am making them template classes. When compiling, I am told that tnode (found in the constructor of the Queue class) has not been declared in the scope. Any suggestions on how to fix this?

EDIT 1: The error message that I am getting is:

RCQueue.h: In constructor ‘Queue::Queue()’:
RCQueue.h:8:28: error: ‘tnode’ was not declared in this scope LinkedList::Node *tnode = Q.getHead();

The main purpose of my constructor is to initialize the "head" pointer from the LinkedList class as NULL. I was also curious as to how one could go about declaring a variable of a structure that was declared in another template class.

Answer 1

Enqueue Algorithm :
1. Create a newNode with data and address.
2. if queue i.e front is empty   
i.  front = newnode;   
ii. rear  = newnode;
3. Else 
i.rear->next = newnode;    
ii.rear = newnode;

Dequeue Algorithm :
1. if queue is i.e front is NULL      printf("\nQueue is Empty \n");
2. Else next element turn into front        
i.  struct node *temp = front ;  
ii. front = front->next;   
iii.free(temp);  

---

C++ implementation :

     #include <bits/stdc++.h>
      using namespace std;

      struct node
      {
        int data;
        node *next;
      };

      node *front = NULL;
      node *rear =NULL;


     void Enque(int data)
     {
           node *newnode = new node;
           newnode->data = data;
           newnode ->next = NULL;


           if(front==NULL) 
           {
               front=newnode;
               rear=newnode;
           }

           else
           {
              rear->next = newnode;
              rear = newnode;
           }
      }





    void Deque()
    {
       struct node *temp;

       if (front == NULL)
       {
         printf("\nQueue is Empty \n");
         return;
       }

       else
       {
           temp = front;
           front = front->next;
           if(front == NULL)  rear = NULL;
           free(temp);
       }
     }


    void display()
    {
        node *temp=front;
        if(front==NULL)
        {
          printf("\nQueue is Empty \n");
        }

        else
        {
            while(temp != NULL)
            {
                cout<<temp->data<<" ";
                temp = temp->next;
            }
        }
        cout<<endl;
    }

Answer 2

You need typename in front of every use of the Node type in LinkedList that you reference within Queue because its dependent on the template parameter T. In specific,

template <class T>
class Queue {

public:
    Queue() {
        typename LinkedList<T>::Node *tnode = Q.getHead();
    }

    ~Queue() {
        Q.MakeEmpty();
    }

    void Enqueue( T x ) {
        typename LinkedList<T>::Node *cnode = Q.getHead();

        //Find the last element of Q
        while( cnode -> next != NULL ) {
            cnode = cnode -> next;
        }
        //Add x to the end of the queue
        Q.Insert( x );
    }

    void Dequeue() {
        typename LinkedList<T>::Node *hnode = Q.getHead();
        //Rest of function
    }

    void Print() {
        Q.PrintList();
    }

private:
    LinkedList<T> Q;
};

Notice the addition of typename before the uses of LinkedList<T>::Node.

Of course you'll also hear complaints about the missing definition of MakeEmpty() within LinkedList that is called in your Queue class, so just add a definition for it.

For more information on why typename is needed, this post explains its pretty clearly.