C++ program crashes while checking if a pointer is NULL using if condition

Question

I was trying a C program on queue as a linked list. Whenever I try to execute, it crashes whenever it encounters if condition comparing a pointer (q->f in this case) with NULL. Please check the following code:

#include<stdio.h>
using namespace std;
struct node
{
    int info;
    node *next;
};
struct que
{
    struct node *f; //front
    struct node *r; //rear
    que()
    {
        f=r=NULL; //initialize as NULL
    }
};
struct que *pq;
/* prototypes */
void disp(struct que *q);
int emp(struct que *q);
void ins(struct que *q,int x);
void del(struct que *q);

int main()
{
    int cho;
    while(1)    //so that it executes continuously and I can exit whenever I want
    {
        printf("Enter 1 to insert in a queue\n");
        printf("Enter 2 to delete in a queue\n");
        printf("Enter 3 to display the queue\n");
        scanf("%d",&cho);
        if (cho==1)
        {
            int x;
            printf("Enter the info to be added\n");
            scanf("%d",&x);
            ins(pq,x);
        }
        else if (cho==2)
            del(pq);
        else if (cho==3)
            disp(pq);
    }
    return 0;
}
int emp(struct que *q) // Check whether queue is empty or not
{
    return ((q->f==NULL)?1:0); //Error
}
void ins(struct que *q,int a)
{
    node *p;
    p=new node;
    p->info=a;
    p->next=NULL;
    if ((q->r)==NULL)   //Error. I get crash and this statement is never executed.
        (q->f)=p;
    else
        (q->r)->next=p;
    (q->r)=p;
    printf("Node added\n");
}
void del(struct que *q)
{
    node *p=NULL;
    if (emp(q))
    {
        printf("Empty queue.Insert some elements\n");
        return;
    }
    p=q->f;
    q->f=p->next;
    delete p;
    printf("Node deleted\n");
}
void disp(struct que *q)
{
    if (emp(q))
    {
        printf("Empty queue.Insert some elements\n");
        return;
    }
    node *i=NULL;
    for (i=q->f;i!=NULL;i=(i)->next)
        printf("%d\n",i->info);
}

I suspect there's something wrong with the statement if ((q->r)==NULL). Executing the program results in a crash " has stopped working". I have also tried replacing it with if (!q->r) , but without much success. I am unable to find a problem in my code.Please help me..Thanks

Answer 1

You never initialize pq, so q->r in the following has undefined behaviour:

if ((q->r)==NULL)   //Error. I get crash and this statement is never executed.

One way to fix this is by turning

struct que *pq;

into

struct que pq;

and then passing &pq into ins() et al.