Thread: Abstract data types in a queue

Hello - I've written a queue and I'm trying to keep it generic as far as the data it could hold. For that purpose, my node structure is defined as such:

Code:

struct node {
void* data;
struct node* next;
}node;

The void* is because I don't want to restrict the data type in the node (int, char, struct). My problem is adding the data to the queue. In my enqueue method, I create a new node to add the data to:

Code:

void enqueue(queue* q, void* item)
{
//....
// Create a new node and assign data
node* new_node = (node*)malloc(sizeof(node));
new_node->data = malloc(sizeof(item));
memcpy(new_node->data, item,strlen((char *)item));
//...
}

My problem is using the memcpy function. I don't know how to copy exactly the size of the data type I intend to pass because of the generality of the code. In the above example I had to use strlen when I wanted to test it with a char*, but I would like to make it work for every data type without having to do a specific cast every time. I'd really appreciate help on this - thanks!

Do you need memcpy? Couldn't you just set new_node->data = item?

I am actually working on the same thing as you right now. And that is the approach I am taking.

I tried that but it referenced the same item in memory for all nodes. So, in my tester.c file I inserted the string "node %d" for i = 0; i < 5, and got:
node 4
node 4
node 4
node 4
node 4

Which drove me crazy. So this is how I'm working around it.

Originally Posted by sa125

I tried that but it referenced the same item in memory for all nodes. So, in my tester.c file I inserted the string "node %d" for i = 0; i < 5, and got:
node 4
node 4
node 4
node 4
node 4

Which drove me crazy. So this is how I'm working around it.

Did you have something like this?

Code:

int i, temp;
for (i = 0; i < 5; i++) {
    printf("Enter number for queue: ");
    scanf("%d", &temp);
    enqueue(queue_ptr, &temp);
}

If you're reusing the same variable when making the queue, then you'll need to malloc and memcpy inside enqueue; if all the data is already distinct in your main program, then you can just assign pointers. (And "passing sizeof data" is already an established idiom in C; that's how the library functions that use void *, like bsearch and qsort, work.)

I did have the same problem as you probably for the reason tabstop mentioned. I was reusing the same variable to add data. Alloc new memory for each piece of data that you add to the queue.