Thread: Problem when concatenating two dynamic queues

  1. #1
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    Problem when concatenating two dynamic queues

    I have a Queue *q1, *q2 in main() that I send off the a function to initialize that looks like this:
    Code:
    Queue *CreateQueue () {
       Queue *queue;
    
       queue->front = queue->rear = NULL;
    
       return queue;
    }
    Doesn't this create a brand new Queue, initialize it to NULL, then return it back to the calling function?

    But what happens is that when I manipulate one queue, the exact same operation is performed on the other queue, as if they were the same queue.

    From my other thread asking for queue help (http://cboard.cprogramming.com/showthread.php?t=57999), itsme86's queue initialization works fine because it creates QUEUE q1, q2; in main () before sending it off to be initialized:
    Code:
    void new_queue(QUEUE *q)
    {
      q->head = q->tail = NULL;
    }
    Why doesn't my method work?

  2. #2
    C++ Developer XSquared's Avatar
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    >>Doesn't this create a brand new Queue, initialize it to NULL, then return it back to the calling function?
    No. You aren't allocating memory for the queue.
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  3. #3
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    Doesn't calling the function to to create a new variable make it separate from one that was created in a previous call? What I'm doing is basically this:
    Code:
    main()
    {
       // .........
    
       Queue *q1 = CreateQueue(q1);
       Queue *q2 = CreateQueue(q2); //why isn't this new q2 separate from q1?
    }
    I then malloc when I append an item onto the queue.

  4. #4
    ~viaxd() viaxd's Avatar
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    I have a Queue *q1, *q2 in main() that I send off the a function to initialize that looks like this:
    Code:
    Queue *CreateQueue () {
       Queue *queue;
    
       queue->front = queue->rear = NULL;
    
       return queue;
    }
    you see, queue is just a pointer, it points to some random location. Here, you try to dereference it w/ -> operator, this is not correct. this is why you should malloc before doing this in CreateQueue() or just pass a pointer to an already existing variable of that type.
    :wq

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