Typecasting a void pointer in a Linked List

Question

I have an array of structs called arrayOfElements and I want them stored ( in better terms pointed to ) in a Linked List so I've malloc'd the arrayOfStrucs

arrayOfElements= malloc(4 * sizeof(Element));

and then once I have put in all the data in I want to pass it to a function called insert which imports the head of the list and the array of structs

LinkedList* insert(LinkedList* head, Element* inArrayOfElements)

My issue is that I've been told that the data member in the linked list has to be a void* , thats a hard requirement. So my question is, in the function insert how do I go about making void* data from the linked list

typedef struct LinkedList {
    void* data
    struct LinkedList* next;
    } LinkedList;

point to the imported array of structs?

LinkedList* insert(LinkedList* head, Element* inArrayOfElements)
{ 
    LinkedList* insertNode = malloc(sizeof(LinkedList));
    insertNode->next = head;
    /*WHAT DO I DO HERE TO MAKE void* data point to inArrayOfELements*/
    return insertNode;
}

insertNode->data = inArrayOfElements? I'm not sure about this — RoadRunner
– RoadRunner, Commented Oct 24, 2016 at 6:18
inserNode->data = (void*)inArrayOfElements. This requirements makes sense, if you will use your linked list to store elements of different types. But then you will need some field to understand what data is in the node. If you store only Element pointers, void* type requirement is rather strange one. — Ari0nhh
– Ari0nhh, Commented Oct 24, 2016 at 6:19
thanks, I think its because each struc in the array will hold a different data type , so ArrayOfElements[1] could be a int and ArrayOfElements[0] could be a string — CrispyCashew
– CrispyCashew, Commented Oct 24, 2016 at 6:24
@RoadRunner actually, typecasting to void* is pointless for this in C. Conversion is done automatically in both directions. Your initial command seems correct, though the question isn't exactly crystal clear. I can't tell whether the OP wants to store each element in inArrayOfElements as a single node in the list (and if so, where's the array size param?), or just a single node with data pointing to the base array pointer. — WhozCraig
– WhozCraig, Commented Oct 24, 2016 at 6:34

Community · Accepted Answer · 2017-05-23 11:48:35Z

3

You can cast to a void pointer, like this:

inserNode->data = (void*)inArrayOfElements;
//                ^ Explicit cast here

This requirement makes a lot of sense if you'll be storing multiple types of data in your data field of the linked list.

Please note that in C, such a cast is optional and only for readability purposes. Conversion from Anything* to void* is strictly speaking unnecessary in C. In C++ it wouldn't, and there are downsides to it, as @WhozCraig points out in his comment. So pick your poison there.

edited May 23, 2017 at 11:48

CommunityBot

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answered Oct 24, 2016 at 6:37

Magisch

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3 Comments

WhozCraig Over a year ago

Conversion from AnyObjectType* to void* and back in C is automatic. No cast should be required.

WhozCraig Over a year ago

Not certainly. It also makes it more brittle. If/when conditions change such as the param is no longer a basic pointer type the cast will happily (and unhelpfully) hide the mistake. With C++ it would be mandetory, but in C there is no need for it, and plenty of reasons to avoid it. Given a choice between using, and not using, an unnecessary cast, choose the latter.

Magisch Over a year ago

@WhozCraig I added a disclaimer to the answer citing your comment. I hope its clearer now.

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