Compilation fails with "expected unqualified-id" on "using"

Question

I need some help making this program for class. We are working with g++ (linux? its through putty on a server) I am having a lot of issue with this linked list. The current error that it is giving me is

queue.cpp:2: error: expected unqualified-id before âusingâ

Wondering if anyone could help me figure it out. A little bit of searching shows that the problem seems to be in the #define somewhere? The error shows that it is in the .cpp file but i think it is in the .h file. Also if you could give me any programing advise about anything that seems off, wrong or if there a better way of doing it.

the queue.h file below

#ifndef QUEUE_H
#define QUEUE_H

template <class Object>
class Queue
{
 public:
     Queue();
     Queue(const Queue& a_queue);
     Queue& operator =(const Queue& rhs);
     bool enqueue(const Object& d);
     bool dequeue(const Object& d);
     bool isEmpty() const;
     ~Queue();

private:
    struct ListNode
    {
        Object obj;
        ListNode *next;

    };
    ListNode *head;
}

#endif //Queue_H
#include "queue.cpp"  //include queue.cpp with file

the queue.cpp file here.

#include <iostream>
using namespace std;
template <class Object>
Queue<Object>::Queue()
{
    head = NULL;
}

template <class Object>
Queue<Object>::Queue(const Queue<Object>& a_queue)
{
    head=NULL;
    *this=a_queue;
}

template <class Object>
Queue<Object>& Queue<Object>::operator =(const Queue<Object> &rhs)
{
    ListNode *nodePtr;
    nodePtr = rhs.head;
    if(this != rhs){
        this.head = NULL;
        while(nodePtr != NULL){
             this.enqueue(nodePtr->obj);
             nodePtr = nodePtr->next;
        }
    }
}

template <class Object>
bool Queue<Object>::enqueue (const Object& d) //Enqueue
{
    ListNode *newNode;
    ListNode *nodePtr;
    ListNode *previousNode;
    previousNode = NULL;
    nodePtr = NULL;
    newNode = new ListNode;
    newNode->obj = d;
    newNode->next = NULL;

    if(isEmpty){
        head = newNode;
        return true;
        }
    else{
        nodePtr = head;
        previousNode = NULL;
        while(nodePtr != NULL){
            previousNode = nodePtr;
            nodePtr=nodePtr->next;
        }
        if(previousNode->next == NULL){
            previousNode->next = newNode;
            return true;
        }
        else
            return false;
    }
}

template <class Object>
bool Queue<Object>::dequeue (const Object& d)  //Dequeue
{
    ListNode *nodePtr;

    if(!head)
        return false;
    else{
        if(head->next != NULL){
            nodePtr = head;
            d=nodePtr->obj;
            head = head->next;
            delete nodePtr;
        }else
            delete head;
        return true;
    }
}

template <class Object>
bool Queue<Object>::isEmpty() const  //Check if Empty
{
    if(!head)
        return true;
    else
        return false;
}

template <class Object>
Queue<Object>::~Queue()   //Destructor
{
    Object temp;
    while (head != NULL)
        dequeue (temp);
}

Answer 1

1. Don't include your implementation file from the header. Include the header from the implementation file.
2. I don't see you say "using namespace std" in the header in your code, but you say in your comment that you do that. Don't. Never say using namespace in a header file.
3. Place all template definitions in the header.
4. You're missing a semicolon in your class definition in the header.
5. Your assignment operator is not exception-safe. Make sure your copy-constructor is correct and then use the copy & swap idiom.
6. Your copy-constructor is incorrect. If you want to support lazy copy (copy on write) then that's fine, but you're missing the actual deep copy operation. Be extra-careful with the copy constructor, because it extremely important you get it right.

Answer 2

You need a semicolon after your class declaration in the header.

class Queue
{
 public:
     Queue();
     Queue(const Queue& a_queue);
     Queue& operator =(const Queue& rhs);
     bool enqueue(const Object& d);
     bool dequeue(const Object& d);
     bool isEmpty() const;
     ~Queue();

private:
    struct ListNode
    {
        Object obj;
        ListNode *next;

    };
    ListNode *head;
};