Question

Why does it crash when I get to the copy constructor function?

In the copy procedure that you will find in my class definitions, I’m making sure that the Queue that will be created as a copy of some other original Queue is empty before copying begins: let’s say Queue q1 is not empty and I want to turn q1 into q2. I want to empty the contents of q1 before copying the contents of q2 to q1..

#include <iostream>
#include <string>
#include <cassert>

using namespace std;

class Dnode
{
    public:
       Dnode(int);
       int  n;
       Dnode*   l, *r;
};

Dnode::Dnode(int tx)
{
    n = tx;
    l = r = NULL;
}

class Queue // reminder: insertions at the rear, deletions at the front
{
    public:
       Queue();
       void enqueue(int x);
       int dequeue(void);
       bool empty(void) const;
       void display(void) const;
       Queue(const Queue&); //copy constructor

    private:
       Dnode*   front, *back;
       void copy(Dnode*);
       void free();

};

Queue::Queue()
{
    front = back = NULL;
}


void Queue::enqueue(int x)
{
    Dnode*  d = new Dnode(x);
    if (empty())
        front = back = d;
    else
    {
        back->r = d;
        d->l = back;
        back = d;
    }
}

int Queue::dequeue(void)
{
    assert(! empty());
    Dnode*  temp = front;
    front = front->r;
    if (front == NULL)
        back = NULL;
    else    front->l = NULL;
    int x = temp->n;
    delete temp;
    return x;
}


bool Queue::empty(void) const
{
    return front == NULL;
}

void Queue::display(void) const
{
    for (Dnode* d = front; d != NULL; d = d->r)
        cout << d->n << " ";
    cout << endl;
}

void Queue::copy(Dnode* dn) // "dn" will be "Front" of Queue being copied
{                           // this procedure will be called in Copy Constructor  
    Dnode* temp=front;      // found underneath this procedure
    while(front!=back)      
    {                       
        front=front->r;
        delete temp;
        temp=front;
    }

    delete temp;

    front=back=temp=NULL;

    if(dn!=NULL)
    {
        while(dn->r!=NULL)
        {
            enqueue(dn->n);
            dn=dn->r;
        }
        enqueue(dn->n);
    }
}

Queue::Queue(const Queue& x)
{
    copy(x.front);
}

int main()
{
    Queue   q;
    if (q.empty()) cout << "q empty" << endl;

    for (int i = 0; i < 10; i++) q.enqueue(i);

    q.display();

    int x = q.dequeue();

    cout << "x is " << x << endl;

    q.display();

    Queue q1(q); //<----program crashes when we get here

    q1.display();
}

Answer 1

You’re confusing a copy constructor with an assignment operator. In an assignment operator, you have to delete the current queue and replace it with a copy of the second parameter. But this is a copy constructor. There is no current queue. Your members are uninitialized. So when you begin by trying to delete the existing queue, you’re messing with uninitialized pointers.