c++ queue template

Question

ALright, pardon my messy code please. Below is my queue class.

#include <iostream>
using namespace std; 
#ifndef QUEUE
#define QUEUE

/*----------------------------------------------------------------------------
Student Class

# Methods #
Student()               // default constructor
Student(string, int)    // constructor
display()               // out puts a student

# Data Members #
Name                    // string name
Id                      // int id
----------------------------------------------------------------------------*/
class Student { 
public: 
    Student() { } 
    Student(string iname, int iid) { 
        name = iname; 
        id = iid; 
    } 
    void display(ostream &out) const { 
        out << "Student Name: " << name << "\tStudent Id: " << id
            << "\tAddress: " << this << endl;  
    }  

private: 
    string name; 
    int id; 
}; 


// define a typedef of a pointer to a student. 
typedef Student * StudentPointer;

template <typename T> 

class Queue
{
public:
    /*------------------------------------------------------------------------
    Queue Default Constructor

    Preconditions: none
    Postconditions: assigns default values for front and back to 0

    description: constructs a default empty Queue. 
    ------------------------------------------------------------------------*/
    Queue() : myFront(0), myBack(0) {}


    /*------------------------------------------------------------------------
    Copy Constructor

    Preconditions: requres a reference to a value for which you are copying
    Postconditions: assigns a copy to the parent Queue. 

    description: Copys a queue and assigns it to the parent Queue. 
    ------------------------------------------------------------------------*/
    Queue(const T & q) { 
        myFront = myBack = 0; 
        if(!q.empty()) { 
            // copy the first node
            myFront = myBack = new Node(q.front()); 
            NodePointer qPtr = q.myFront->next; 
            while(qPtr != NULL) { 
                myBack->next = new Node(qPtr->data); 
                myBack = myBack->next; 
                qPtr = qPtr->next; 
            } 
        } 

    }
    /*------------------------------------------------------------------------
    Destructor

    Preconditions: none
    Postconditions: deallocates the dynamic memory for the Queue

    description: deletes the memory stored for a Queue. 
    ------------------------------------------------------------------------*/
    ~Queue() { 
        NodePointer prev = myFront, ptr; 
        while(prev != NULL) { 
            ptr = prev->next; 
            delete prev; 
            prev = ptr; 
        } 
    } 
    /*------------------------------------------------------------------------
    Empty()

    Preconditions: none
    Postconditions: returns a boolean value. 

    description: returns true/false based on if the queue is empty or full. 
    ------------------------------------------------------------------------*/
    bool empty() const { 
        return (myFront == NULL); 
    }
    /*------------------------------------------------------------------------
    Enqueue

    Preconditions: requires a constant reference
    Postconditions: allocates memory and appends a value at the end of a queue

    description: 
    ------------------------------------------------------------------------*/
    void enqueue(const T & value) {
        NodePointer newNodePtr = new Node(value); 
        if(empty()) { 
            myFront = myBack = newNodePtr; 
            newNodePtr->next = NULL; 
        } else { 
            myBack->next = newNodePtr; 
            myBack = newNodePtr; 
            newNodePtr->next = NULL; 
        } 
    }
    /*------------------------------------------------------------------------
    Display

    Preconditions: requires a reference of type ostream
    Postconditions: returns the ostream value (for chaining)

    description: outputs the contents of a queue. 
    ------------------------------------------------------------------------*/
    void display(ostream & out) const {
        NodePointer ptr; 
        ptr = myFront; 

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

    Preconditions: none
    Postconditions: returns a value of type T

    description: returns the first value in the parent Queue. 
    ------------------------------------------------------------------------*/
    T front() const {
       if ( !empty() ) 
          return (myFront->data);
       else
       {
          cerr << "*** Queue is empty -- returning garbage value ***\n";
          T * temp = new(T); 
          T garbage = * temp;
          delete temp; 
          return garbage;
       }
    }
    /*------------------------------------------------------------------------
    Dequeue

    Preconditions: none
    Postconditions: removes the first value in a queue
    ------------------------------------------------------------------------*/
    void dequeue() {
        if ( !empty() ) { 
            NodePointer ptr = myFront; 
            myFront = myFront->next; 
            delete ptr; 
            if(myFront == NULL) 
                myBack = NULL; 

        } else {
            cerr << "*** Queue is empty -- "
              "can't remove a value ***\n";
            exit(1);
        }
    }
    /*------------------------------------------------------------------------
    pverloaded = operator

    Preconditions: requires a constant reference
    Postconditions: returns a const type T

    description: this allows assigning of queues to queues
    ------------------------------------------------------------------------*/
    Queue<T> & operator=(const T &q) { 
    // make sure we arent reassigning ourself
    // e.g. thisQueue = thisQueue. 
        if(this != &q) { 
            this->~Queue(); 
            if(q.empty()) { 
                myFront = myBack = NULL; 
            } else { 
                myFront = myBack = new Node(q.front()); 
                NodePointer qPtr = q.myFront->next; 
                while(qPtr != NULL) { 
                    myBack->next = new Node(qPtr->data); 
                    myBack = myBack->next; 
                    qPtr = qPtr->next; 
                } 
            } 
        } 
        return *this; 
    }

private:
    class Node { 
    public: 
        T data; 
        Node * next; 
        Node(T value, Node * first = 0) : data(value),
                                          next(first) {}

    };  
    typedef Node * NodePointer; 

    NodePointer myFront,
               myBack,
               queueSize; 


}; 

/*------------------------------------------------------------------------
join

Preconditions: requires 2 queue values
Postconditions: appends queue2 to the end of queue1

description: this function joins 2 queues into 1. 
------------------------------------------------------------------------*/

template <typename T>
Queue<T> join(Queue<T> q1, Queue<T> q2) {
    Queue<T> q1Copy(q1), q2Copy(q2); 
    Queue<T> jQueue; 


    while(!q1Copy.empty()) { 
        jQueue.enqueue(q1Copy.front()); 
        q1Copy.dequeue(); 
    } 

    while(!q2Copy.empty()) { 
        jQueue.enqueue(q2Copy.front()); 
        q2Copy.dequeue(); 
    } 
    cout << jQueue << endl; 
    return jQueue;   

} 
/*----------------------------------------------------------------------------
Overloaded << operator 

Preconditions: requires a constant reference and a Queue of type T
Postconditions: returns the ostream (for chaining)

description: this function is overloaded for outputing a queue with <<
----------------------------------------------------------------------------*/
template <typename T>
ostream & operator<<(ostream &out, Queue<T> &s) { 
    s.display(out);  
    return out; 
} 

/*----------------------------------------------------------------------------
Overloaded << operator

Preconditions: requires a constant reference and a reference of type Student
Postconditions: none

description: this function is overloaded for outputing an object of type
             Student. 
----------------------------------------------------------------------------*/
ostream & operator<<(ostream &out, Student &s) { 
    s.display(out); 
}

/*----------------------------------------------------------------------------
Overloaded << operator

Preconditions: requires a constant reference and a reference of a pointer to
               a Student object. 
Postconditions: none

description: this function is overloaded for outputing pointers to Students
----------------------------------------------------------------------------*/
ostream & operator<<(ostream &out, StudentPointer &s) { 
    s->display(out); 
}
#endif

Now I'm having some issues with it. For one, when I add 0 to a queue and then I output the queue like so..

Queue<double> qdub; 
qdub.enqueue(0); 
cout << qdub << endl; 

That works, it will output 0. But for example, if I modify that queue in any way.. like.. assign it to a different queue..

Queue<double> qdub1; 
Queue<double> qdub2; 
qdub1.enqueue(0; 
qdub2 = qdub1; 
cout << qdub2 << endl; 

It will give me weird values for 0 like.. 7.86914e-316.

Help on this would be much appreciated!

Answer 1

Your haven't defined a copy constructor or an assignment operator. The ones you have take an instance of the queued type, not another queue. For assigning and copying queues themselves, the compiler will still use the automatically generated ones which do the wrong thing.

(This probably doesn't explain the output of that particular snippet.)

Another thing that is completely wrong (even though, again, the snippet never invokes this function or you'd get compiler errors all over the place):

Queue<T> & operator=(const T &q) {
// make sure we arent reassigning ourself
// e.g. thisQueue = thisQueue.
    if(this != &q) {
        this->~Queue();

Calling destructor explicitly like that, and then going on to use the instance is not allowed. Explicit destructor calls only go hand in hand with constructing objects with placement new.

operator= is normally implemented in terms of copy constructor and a swap method (which swaps the internal representation between two instances):

void swap(Queue<T>& rhv)
{
   std::swap(myFront, rhv.myFront);
   std::swap(myBack, rhv.myBack);
   std::swap(queueSize, rhv.queueSize);
}

Queue<T>& operator=(const Queue<T>& rhv)
{
   Queue<T> copy(rhv);
   this->swap(copy);
} //the destructor of copy releases the previous contents of *this

Answer 2

I don't see an assignment operator there, which means you're getting the compiler generated default, which will do a shallow copy. You probably need to supply your own to do a deep copy instead. What your comments call a copy ctor isn't really a copy ctor either. A copy ctor always takes a const reference to the object being copied, so the signature would be: Queue(Queue const &original);.

Answer 3

You need a proper assignment operator. Your example would probably not compile the way you provided your class.

Even if I am wrong, the main mistake in your code is that your operator= calls it's own destructor. This is horribly wrong. The destructor will later called 'naturally' on as well. This means that your objects will be deleted twice. (Because you don't assign NULL to Queue.myFront in your destructor.)

Don't manually call destructors.

For a basic exercise, I recommend that you place a breakpoint on the line qdub2 = qdub1 and then debug step by step to see what your code really does.