Pointers to functions casting

Question

I have the following structs:

struct mtmFlix_t {
    List usersList;
    List seriesList;
};

struct User_t {
    int age;
    char* username;
    MtmFlix mtmFlix;
};

These are the typedefs in list.h :

typedef void* ListElement;
typedef ListElement(*CopyListElement)(ListElement);
typedef void(*FreeListElement)(ListElement);

These are the typedefs in user.h and MtmFlix.h :

typedef struct User_t *User;
typedef struct mtmFlix_t* MtmFlix;

I would like to use the following function in mtmflixCreate, but I can't seem to figure out how to cast the UserCreate and UserFree to (*void) ?

List listCreate(CopyListElement copyElement, FreeListElement freeElement);

MtmFlix mtmFlixCreate()
{
    MtmFlix newMtmFlix = malloc(sizeof(*newMtmFlix));
    if (newMtmFlix == NULL) {
        return NULL;
    }
    newMtmFlix->seriesList=listCreate(?????);
    newMtmFlix->usersList=listCreate(?????);
}

The following functions appear in user.h :

User UserCreate(MtmFlix mtmFlix, const char* username,int age);
Void UserFree(User user);

C requires exact matches on the function signature, not "close enough". Your functions don't match. — tadman, May 10 '18 at 23:09
Possible duplicate of [Casting a function pointer to another type](https://stackoverflow.com/questions/559581/casting-a-function-pointer-to-another-type) — Increasingly Idiotic, May 10 '18 at 23:22
[Is it a good idea to typedef pointers?](https://stackoverflow.com/questions/750178/is-it-a-good-idea-to-typedef-pointers) — Barmar, May 10 '18 at 23:45
Am I missing the obvious, or does the code not show the definition of the type `List`? — Jonathan Leffler, May 11 '18 at 00:10
@Yasmin12 "can't seem to figure out how to cast the UserCreate and UserFree to (*void)" --> Do you mean to `void*`, a `void` pointer? That is usually a weak approach as a function pointer may not fit in `void *`. — chux - Reinstate Monica, May 11 '18 at 00:20

score 1 · Accepted Answer · answered May 11 '18 at 03:45

You don't. You have to create functions that have the needed types. Something like this:

ListElement CopyUserListElement(ListElement elem) {
    // (ListElement) is not necessary here, but included for completeness
    return (ListElement)CopyUser((User_t*)elem);
}

void FreeUserListElement(ListElement elem) {
    UserFree((User_t*)elem);
}

Pointers to functions casting

1 Answers1