Abstract data types in a queue

sa125 · 07-27-2008, 12:30 AM

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!

**laserlight** · 07-27-2008, 12:42 AM

You could add another parameter to specify the size of the data.

noops · 07-27-2008, 08:46 AM

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.

sa125 · 07-27-2008, 10:37 AM

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.

tabstop · 07-27-2008, 11:22 AM

Quote:

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.)

noops · 07-27-2008, 02:51 PM

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.

07-27-2008, 12:30 AM	#1
sa125 Registered User Join Date: Jul 2008 Posts: 16	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!
sa125 is offline

07-27-2008, 12:42 AM	#2
laserlight C++ Witch Join Date: Oct 2003 Location: Singapore Posts: 10,368	You could add another parameter to specify the size of the data. __________________ C + C++ Compiler: MinGW port of GCC Build + Version Control System: SCons + Bazaar Look up a C/C++ Reference and learn How To Ask Questions The Smart Way
laserlight is online now

07-27-2008, 08:46 AM	#3
noops * Join Date: Jun 2008 Posts: 108	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.
noops is offline

07-27-2008, 10:37 AM	#4
sa125 Registered User Join Date: Jul 2008 Posts: 16	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.
sa125 is offline

07-27-2008, 02:51 PM	#6
noops * Join Date: Jun 2008 Posts: 108	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.
noops is offline

Thread Tools
Show Printable Version Email this Page
Display Modes
Linear Mode Switch to Hybrid Mode Switch to Threaded Mode

Similar Threads
Thread	Thread Starter	Forum	Replies	Last Post
reading binary file with different data types	larne	C Programming	8	07-29-2008 10:12 AM
data structure design for data aggregation	George2	C# Programming	0	05-20-2008 06:43 AM
Need help with ADT (abstract data types)	ortegac	C Programming	1	03-30-2006 02:23 AM
Possible to pack smaller data types into larger ones?	Heraclitus	C Programming	3	02-09-2003 03:22 PM
C Programming Question	TK	A Brief History of Cprogramming.com	13	07-04-2002 07:11 PM