Print void pointer in a Linked List

Question

I have an array of structs called arrayOfElements , the structs are called Elements which have a void pointer

typedef struct {
    void* data;
} Element;

Ive malloc'd arrayOfElements

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

and have stored ints and strings in the strucs

arrayOfElements[3].data = malloc( sizeof(int) );
ptr = arrayOfElements[3].data;
*ptr = 65;

strcpy(arrayOfElements[1].data, token );

Then I have created a Linked List

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

And have created a function to import a instance of arrayOfELements and make void* arrayOfStruct point to it

LinkedList* insert(LinkedList* head, Element* inArrayOfElements) { 
    LinkedList* insertNode = malloc(sizeof(LinkedList));

    insertNode->next = head;
    insertNode->arrayOfStruct = (void*)inArrayOfElements;

    return insertNode;
}

Question

My question is after I have pointed void* arrayOfStruc to an instance of arrayOfELements, how do I print void* data of arrayOfElements[3] which I know from before is an int and its value is 65

Current Node in LinkedList ---> arrayOfElements[3] --> data member in Element struct (should equal 65)

I have a feeling I should point a int pointer to it and then print that, but Im not sure the syntax to do it

dbush · Accepted Answer · 2016-10-27 03:36:55Z

2

You need some way of knowing what kind of data the data member points to. An additional element denoting the type would work.

typedef enum {
    STR_TYPE,
    INT_TYPE,
    ...
} data_type;

typedef struct {
    data_type type;
    void* data;
} Element;

Then you can set the element like this:

arrayOfElements[3].type = INT_TYPE;
arrayOfElements[3].data = malloc( sizeof(int) );
int *ptr = arrayOfElements[3].data;
*ptr = 65;

arrayOfElements[1].type = STR_TYPE;
strcpy(arrayOfElements[1].data, token );

and read it like this:

Element *curr = &list_node->arrayOfStruct[3];
if (curr->type == INT_TYPE) {
    int *pint = curr->data;
    printf("int data = %d\n", *pint);
}

answered Oct 27, 2016 at 3:36

dbush

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1 Comment

CrispyCashew Over a year ago

Thanks for that, i didnt even think of setting a type variable. Is &list_node a pointer to the current node im in? ( or what ever node I want to read from) , so then curr points to arrayOfStruct[3] ?

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