Question

I am trying to make a simple Queue class. Now I am stuck. Its almost complete. I have figured out that only one function “pop” is causing the problem. Can anyone please tell me what should I do.

Here is the code:
(queue.h)

#ifndef  _QUEUE_
#define  _QUEUE

#include <iostream>
#include <string>
using namespace std;


struct Stuff
{
    string name;
    int roll;
};


class Queue
{
private:

struct Node
{
    Stuff data ;
    struct  Node * next;
};
Node *front;
Node *back;
int qsize;
const int MAX;
public:

Queue(int size = 5);
~Queue();

bool isfull() const;
bool isempty() const;
int queuesize() const;

bool push(const Stuff & item);
bool pop();
Stuff first();
Stuff last();


};




#endif

queue.cpp

#include <iostream>
using namespace std;
#include "queue.h"

Queue::Queue(int size)
:MAX(size)
{
front = back = 0;
qsize = 0;

}
bool Queue::isempty() const
{
if(qsize == 0)
    return true;
else return false;
}
bool Queue::isfull() const
{
if(qsize == MAX)
    return true;
else 
    return false;
}
Queue::~Queue()
{
Node * temp;
while(front != NULL)
{
    temp = front;
    front = front->next;
    delete temp;
}

}
int Queue::queuesize() const
{
return qsize;
}

bool Queue::push(const Stuff & swag)
{
if( isfull() )
    return false;

Node *add = new Node;

if(add == NULL)
    return false;
add->data = swag;  
add->next = NULL;

if(front == NULL)
    front = add;
else
    back->next = add->next;
back = add;
qsize++;

return true;
}


bool  Queue::pop()
{
if(isempty() )
    return false;
if(front == NULL)
    return false;
Node *temp =front;


// I think this part is doing something wrong.
front = front->next;



delete temp;
qsize--;
if(qsize == 0)
    back = NULL;
return true;

}
Stuff Queue::first()
{
return front->data;
}
Stuff Queue::last()
{
return back->data;
}

main.cpp

 #include <iostream>
 #include "queue.h"
 #include <string>
 using namespace std;

 int main()
 {
Queue a(5);
Stuff data;
data.name = "icp";
data.roll= 755;
a.push(data);
Stuff x = a.last();

cout << x.name << "\t" << x.roll << endl;
data.name = "sms";
data.roll= 12544;
a.push(data);
x = a.last();   
cout << x.name << "\t" << x.roll << endl;

data.name = "jmc";
data.roll= 98740;
a.push(data);
x = a.last();
cout << x.name << "\t" << x.roll << endl;

cout << a.queuesize() << endl;

/////////////
x = a.first();  
cout << x.name << "\t" << x.roll << endl;
a.pop();

x = a.first();  
cout << x.name << "\t" << x.roll << endl;
a.pop();

x = a.first();  
cout << x.name << "\t" << x.roll << endl;
a.pop();


//// 
cin.get();
cin.get();
return 0;
}

The Program crashes after popping out the first element.
I have pointed out the part that I think is making some problem. Thanks in advance 🙂

Answer 1

I think the problem is caused by push function.
the following code

if(front == NULL)
    front = add;
else
    back->next = add->next;

back->next is set to be add->next, which is NULL. add->next = NULL;

the add is next one to back, so it should be back->next = add;

You think the problem in pop function is because the front is NULL, so front->next is wrong.

Hope this could help you.