Question

recently, while reading former’s code in my current project, I encounter the problems below:

while implementing the Queue, my former wrote codes like this:

while(uq->pHead)
{
    char *tmp = uq->pHead;
    uq->pHead = *(char **)tmp;
    //...
}

the uq->pHead has definition like:

typedef struct {
        char* pHead;
        //...
} Queue;

Well, I’m quite confused about the usage that “uq->pHead = *(char**)tmp” , could anyone explain it to me in detail?

if we assume that *(uq->pHead) = 32(i.e. ‘ ‘) , *(char**)tmp would translate this into pointer-form, but…how could it make sense?

Thanks a lot.

Answer 1

Let’s suppose that we’re implementing your Queue as a linked list. We might have:

struct data_type;

struct node
{
    node *next;
    data_type item;
};

struct linked_list
{
    node *pHead;
    // ...
};

To empty the linked list, we might write:

linked_list *uq=...;
while (uq->pHead)
{
    // unlink the first node from the list
    node *tmp = uq->pHead;
    uq->pHead = tmp->next; 

    // do something with that node
    // ...

    // deallocate the node
    free(tmp);
}

Now suppose we don’t really care about maintainable code, or are otherwise being lazy. We might instead just figure any pointer would do, and keep the structure for ‘node’ in our head, and write:

linked_list *uq=...;
while (uq->pHead)
{
    // unlink the first node
    char *tmp = uq -> pHead;     // tmp points to the first 'node'
    uq -> pHead = *(char**)tmp;  // The first thing in a 'node' is a pointer to
                                 // the next node.

    // do something with 'tmp', the now unlinked node
    data_type *item=(data_type*) ( ((char**)tmp) + 1 ); // after the 'next' pointer
                                                        // is the real data.
    // ...

    // free up the node
    free(tmp);
}