C++ Queue from Linked List - c++

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.