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Segmentation Fault?

Nov 3, 2011 at 3:01am
saverman12 (8)
Why I am getting a segmentation fault in my program? I think its a problem with my push function being stuck in the decision statement. How can I fix this?

Here is my code: 
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#ifndef QUEUE_H
#define QUEUE_H

#include <iostream>
#include <stdexcept>
#include <cstdlib>

template <class T>
struct QNode
   {
     QNode(const T&);

     //Data members
     T data;
     QNode<T> *next;
   };


//Method definitions for the QNode class

template <class T>
QNode<T>::QNode(const T& newData)
{  
  data = newData
  next = NULL;  
}

// Forward declaration of the Queue template class
template <class T>
class Queue;

// Forward declaration of the operator<< template function
template <class T>
std::ostream& operator<<(std::ostream&, const Queue<T>&);

template <class T>
class Queue
   {
   // friend declaration for the template function - note the
   // special syntax
   friend std::ostream& operator<< <>(std::ostream&, const Queue<T>&);

   public:

        Queue();
      // ~Queue();
       // Queue(const Queue<T>&) copy constructor?
        int size() const;
        bool empty() const;
       // void clear();
       // T& front() throw(out_of_range);
       // T& back() throw(out_of_range);
        void push(const T&);
       // void pop();


    //Operators to overload
    //assignment operator

   private:

         QNode<T> *qFront;
         QNode<T> *qRear;

   };

// Method definitions for the Queue class

template <class T>
std::ostream& operator<<(std::ostream& out, const Queue<T>& rightOp)
                                                                    
template <class T>
class Queue
   {
   // friend declaration for the template function - note the
   // special syntax
   friend std::ostream& operator<< <>(std::ostream&, const Queue<T>&);

   public:

        Queue();
      // ~Queue();
       // Queue(const Queue<T>&) copy constructor?
        int size() const;
        bool empty() const;
       // void clear();
       // T& front() throw(out_of_range);
       // T& back() throw(out_of_range);
        void push(const T&);
       // void pop();


    //Operators to overload
    //assignment operator

   private:

         QNode<T> *qFront;
         QNode<T> *qRear;

   };

// Method definitions for the Queue class

template <class T>
std::ostream& operator<<(std::ostream& out, const Queue<T>& rightOp)

{
  int i;
  for(i = 0; i < rightOp.size(); i++)
     out << " " << rightOp;
  
  return out;
}                                               

template <class T>
Queue<T>::Queue()
{
  qFront = NULL;
  qRear  = NULL;
}

template <class T>
int Queue<T>::size() const
{
  int count = 0;
  QNode<T> *ptr;

  ptr = qFront;

  while(ptr != NULL)
  {
    count++;
    ptr = ptr->next;
  }

 return count;

}

template <class T>
bool Queue<T>::empty() const
{
  if (qFront == NULL)
     return true;
  else
     return false;
}

template <class T>
void Queue<T>::push(const T& item)
{
  QNode<T> *ptr, *lagPtr;
  ptr = new QNode<T>(item);

  ptr->data = item;
  ptr->next = NULL;

   if(qFront == NULL)
  {
    qFront = ptr;
  }
  else
  {
    lagPtr = qFront;
      while(lagPtr->next!=NULL)
      lagPtr = lagPtr->next;

   lagPtr->next = ptr;
  }


}

#endif /* QUEUE_H */


Driver Program

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#include "Queue.h"
#include <iostream>
#include <stdexcept>

using std::cout;
using std::endl;
using std::out_of_range;

int main()
   {
   cout << "Testing default constructor\n\n";

   Queue<int> q1;

   cout << "q1 (size " << q1.size() << "): " << q1 << endl;
   cout << "q1 is " << ((q1.empty()) ? "empty\n" : "not empty\n");
   cout << endl;

   cout << "Testing push()\n\n";

   q1.push(17);

   cout << "q1 (size " << q1.size() << "): " << q1 << endl; //causes                                                
                                                          // segmentation fault
   cout << "q1 is " << ((q1.empty()) ? "empty\n" : "not empty\n");
   cout << endl;
Edit & run on cpp.sh



Last edited on Nov 3, 2011 at 2:27pm
Nov 3, 2011 at 3:26am
shacktar (1187)
Well, this depends on the T. If it allows assignment, then all you have to do is this:

data = newData;
Nov 3, 2011 at 3:54am
saverman12 (8)
I don't believe it works because my push function isn't working properly. Or maybe my size function.

Here is all my code so far:

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#ifndef QUEUE_H
#define QUEUE_H

#include <iostream>
#include <stdexcept>
#include <cstdlib>

template <class T>
struct QNode
   {
     QNode(const T&);

     //Data members
     T data;
     QNode<T> *next;
   };


//Method definitions for the QNode class

template <class T>
QNode<T>::QNode(const T& newData)
{
   data = newData;
   next = NULL;
}


template <class T>
class Queue;

// Forward declaration of the operator<< template function
template <class T>
std::ostream& operator<<(std::ostream&, const Queue<T>&);

template <class T>
class Queue
   {
   // friend declaration for the template function 
   // special syntax
   friend std::ostream& operator<< <>(std::ostream&, const Queue<T>&);

   public:

        Queue();
      // ~Queue(); 
       // Queue(const Queue<T>&) copy constructor
        int size() const;
        bool empty() const;
       // void clear();
       // T& front() throw(out_of_range);
       // T& back() throw(out_of_range);
        void push(const T&);
       // void pop();


    //Operators to overload
    //assignment operator

   private:

         QNode<T> *qFront;
         QNode<T> *qRear;

   };


template <class T>
std::ostream& operator<<(std::ostream& out, const Queue<T>& rightOp)

{
  int i;
  for(i = 0; i < rightOp.size(); i++)
     out << " " << rightOp; // is this correct? 

  return out;
}



template <class T>
Queue<T>::Queue()
{
  qFront = NULL;
  qRear  = NULL;
}

template <class T>
int Queue<T>::size() const
{
  int count = 0;
  QNode<T> *ptr;

  ptr = qFront; //trans list

//count nodes
  while(ptr != NULL) 
  {
    count++;
    ptr = ptr->next;
  }

 return count;

}

template <class T>
bool Queue<T>::empty() const
{
  if (qFront == NULL)
     return true;
  else
     return false;
}

template <class T>
void Queue<T>::push(const T& item)
{
  QNode<T> *ptr, *lagPtr;
  ptr = new QNode<T>(item); //If I don't put(item) i get an error regarding the QNode construct

  ptr->data = item;
  ptr->next = NULL;

  if(qFront == NULL)
  {
    qFront = ptr;
  }
  else
  {
    lagPtr = qFront;
      while(ptr->next!=NULL)
      lagPtr = lagPtr->next;

   lagPtr->next = ptr;
  }

}

#endif


Driver program

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#include "Queue.h"
#include <iostream>
#include <stdexcept>

using std::cout;
using std::endl;
using std::out_of_range;

int main()
   {
   cout << "Testing default constructor\n\n";

   Queue<int> q1;

   cout << "q1 (size " << q1.size() << "): " << q1 << endl;
   cout << "q1 is " << ((q1.empty()) ? "empty\n" : "not empty\n");
   cout << endl;

   cout << "Testing push()\n\n";

   q1.push(17);

   cout << "q1 (size " << q1.size() << "): " << q1 << endl;
   cout << "q1 is " << ((q1.empty()) ? "empty\n" : "not empty\n");
   cout << endl;
Edit & run on cpp.sh


If I comment out the size function in the driver program it will "compile" but I don't know if its really the size function being the problem or my push function. I am really new with templates, queues, and linked lists.
Nov 3, 2011 at 4:02am
shacktar (1187)
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else
{
    lagPtr = qFront;
        while(ptr->next!=NULL)
        lagPtr = lagPtr->next;

    lagPtr->next = ptr;
}

You meant lagPtr->next!=NULL instead in your loop condition.
Last edited on Nov 3, 2011 at 4:02am
Nov 3, 2011 at 4:28am
saverman12 (8)
I still get a segmentation fault with the changed you mentioned. I think its stuck in the decision statement. Is it because the list is some how never reaches NULL?
Last edited on Nov 3, 2011 at 5:18am
Nov 3, 2011 at 5:28pm
shacktar (1187)
Here is the problem:

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template <class T>
std::ostream& operator<<(std::ostream& out, const Queue<T>& rightOp)

{
  int i;
  for(i = 0; i < rightOp.size(); i++)
     out << " " << rightOp; // is this correct?  /*No, it isn't*/

  return out;
}

This enters into a recursive call, printing the Queue over and over. The reason this didn't happen at the beginning is because the size was zero.

What you want to do is start at the beginning of the queue and loop over it, much like you're already doing in the push and size methods, while printing each node's data.
Nov 3, 2011 at 6:32pm
saverman12 (8)
Yeah this seems to work:

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template <class T>
std::ostream& operator<<(std::ostream& out, const Queue<T>& rightOp)
{
  QNode<T> *ptr;
  ptr = rightOp.qFront;

  while(ptr != NULL)
  {
     out << " " << ptr->data;
     ptr = ptr->next;
  }


  return out;
}


Thanks for the help


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