Question

I have a list in which i want to be able to put different types. I have a function that returns the current value at index:

void *list_index(const List * list, int index) {
    assert(index < list->size);
    return list->data[index];
}

In the array there are multiple types, for example:

typedef struct structA { List *x; char *y; List *z; } structA;
typedef struct structB { List *u; char *w; } structB;

Now in order to get data from the array:

structA *A;
structB *B;
for(j=0... ) {
 A = list_index(list, j);
 B = list_index(list, j);
}

But now how do I find out the type of the return value? Is this possible with typeof (I’m using GCC btw)?

And is this even possible or do i have to make some sort of different construction?

Answer 1

C handles types and typing entirely at compile time (no dynamic typing), so once you’ve cast a pointer to a ‘void *’ its lost any information about the original type. You can cast it back to the original type, but you need to know what that is through some other method.

The usual way to do this is with some kind of type tag or descriptor in the beginning of all the objects that might be stored in your list type. eg:

typedef struct structA { int tag; List *x; char *y; List *z; } structA;
typedef struct structB { int tag; List *u; char *w; } structB;
enum tags { structAtype, structBtype };

You need to ensure that every time you create a structA or a structB, you set the tag field properly. Then, you can cast the void * you get back from list_index to an int * and use that to read the tag.

void *elem = list_index(list, index)
switch (*(int *)elem) {
case structAtype:
    /* elem is a structA */
              :
case structBtype:
    /* elem is a structB */