Queue Linked List

[zagzag]

Hi I am doing this project working with linked lists ,however everything works fine until I try my Clear() function which is suppose to clear the queue and de allocate the pointers. However when I do that I get access violation error. Keep in mind that my test harness is a simple test to see if Clear() function works
here is my code in the header file
Take a look at my Clear() function

#ifndef QUEUE_H
#define QUEUE_H

#include<iostream>
#include<cstdlib>
#include<iomanip>



template< typename T >
class Queue
{
public:
	~Queue();
	Queue();
	 Queue(const Queue&);

	void Push(const T& t);
	T Pop();
	bool Empty() const;
	
	T& Front() ;
	size_t Size() const;
	void Clear();
	void Display(std::ostream& os ,char ofc = '\0')const;
	Queue& operator = (const Queue&);
private:
	class Link
	{
		Link ( const T& t) :element_(t),nextLink_(0){}
		T element_ ;
		Link * nextLink_ ;
		friend class Queue<T> ;
	
	};
	Link * firstLink_ ;
	Link * lastLink_ ;
	void         Copy   (const Queue<T>& q);

};
template< typename T >                                                               
Queue<T>::~Queue()                                                                   
{                                                                                    
                                                                                     
  if(!Empty())                                                                       
  {                                                                                  
    Link * currentLink = firstLink_;                                                 
    Link * tempLink;                                                                 
    while(currentLink != 0)                                                          
    {                                                                                
      tempLink = currentLink;                                                        
    currentLink = currentLink ->nextLink_;                                           
    delete tempLink;                                                                 
    }                                                                                
  }                                                                                  
                                                                                     
                                                                                     
}
template<typename T>
Queue<T>::Queue(const Queue& q)                                                                                                                                                   
	:firstLink_(0),                                                                                                                                                      
	lastLink_(0)                                                                                                                                                         
{                                                                                                                                                                                 
                                                                                                                                                                                  
Copy(q);                                                                                                                                                                      
} 
template<typename T>
Queue<T>::Queue()
	:firstLink_(0),
	lastLink_(0)
{}

template<typename T>
void Queue<T>::Push(const T& t)
{
	Link * newLink = new Link(t);
	if(firstLink_ == 0 && lastLink_ ==0)
	{
		firstLink_ = lastLink_ = newLink;
	}
	else
lastLink_ -> nextLink_= newLink;
 lastLink_ = newLink;
	
}

template<typename T>
void Queue<T>::Display(std::ostream& os, char ofc ) const
{

Link * currentLink = firstLink_;
os<<"Queue size: " <<Size();


os<<"Queue content: ";
while(currentLink != 0)
{

	os<<currentLink ->element_;

	currentLink = currentLink ->nextLink_;
}
}
template< typename T >
T Queue<T>::Pop()
{
	

	T removedElement;

Link * temp = firstLink_;


if(firstLink_ == lastLink_)
{
	firstLink_ = lastLink_ = 0;
}

firstLink_ = firstLink_ -> nextLink_;

removedElement = temp -> element_;
delete temp;
return removedElement;

}
template< typename T >
T& Queue<T>::Front() 
{
	return firstLink_ ->element_;
}


template< typename T >
size_t Queue<T>::Size() const
{
T size = 0;

Link * currentLink = firstLink_;

while(currentLink != 0)
{
	
	currentLink = currentLink->nextLink_;
	size++;
}
return size;
}
template<typename T>
bool Queue<T>::Empty() const
{
	return firstLink_ == 0;
}

template< typename T >
void Queue<T>::Clear()
{

	Link * currentLink = firstLink_ ;                                              
    Link * tempLink;                                                                 
         
	while(currentLink)
	{

	tempLink = currentLink;
	currentLink = currentLink ->nextLink_;
    delete tempLink; 

	}
	
}

template<typename T >
Queue<T>& Queue<T>::operator = (const Queue& q)
	
{
 if(this != &q)
 {
	Clear();
Copy(q);

 }
	return *this;
	

}

template < typename T > 
  void Queue<T>::Copy (const Queue<T>& q)
  {

    if (firstLink_ != 0)
    {
      std::cerr << "** Error: Queue::Copy called by non-empty object\n";
        exit(EXIT_FAILURE);
    }

    if (q.firstLink_ == 0)
      return;

    Link* qlink = q.firstLink_;
    while (qlink != 0)
    {

      Push(qlink -> element_);
      qlink = qlink -> nextLink_;

    }
  }
#endif 

here is my main file which has a simple test

#include"queue.h"



int main() 
{

	Queue<int> obj;
	Queue<int>obj2;

	obj.Push(5);
	obj.Push(6);
	obj.Push(32);
	obj.Clear();
	obj.Display(std::cout);
	std::cout<<std::endl;
        obj2 = obj;
	obj.Display(std::cout);


	
}

What may be Causing the error ? Help please

[heebleworp]

I believe the problem is that you need to set firstLink_ equal to zero after you clear everything. Probably should do the same with lastLink_ as well.

[zagzag]

Wow that sovled it ! THANK U! u are a genius !

[lie20]

I have a problem when I got the task from my lecture

task: Made a program about queue in Bank teller system. Teller A if deposit <=10million, teller B > 10million
the count of customer(queue) is random and the price if deposit is random too.

output: how much customers,time and total transaction at teller A, also at teller B.

I hope someone can solve my probelm, Thx