How to use enum within a struct in ANSI C?

问题:

Following code has to be used in the main-function, but I don't know how it is used.

struct SomeItem
{
    enum {MOVIE, MUSIC} itemType;
    union {
        struct Movie* movie;
        struct Music* music;
    };
};

this struct is used in a dynamic linked list with previous/item/next pointer, but I don't know how you can set the enum. Or how to initialize it.

I need to know how it would look like in the main-function.

biglist.someitem = ???;
/* declaration I use */
struct Library* biglist;

more code to understand what Im trying to do.

struct Library{
struct SomeItem* someitem;
struct SomeItem* previousItem;
struct SomeItem* nextItem;
};

compiler errors: C2037: left of 'someitem' specifies undefined struct/union 'library' C2065: MOVIE: undeclared identifier

Im still a rookie on ANSI C, so dont shoot me ok ;)

回答1:

You're using pointers everywhere, so you need to use -> to reference the items.

ie. biglist->someitem->itemType = MOVIE;

The below code compiles fine with gcc -Wall -strict:

struct SomeItem
{
    enum {MOVIE, MUSIC} itemType;
    union {
        struct Movie* movie;
        struct Music* music;
    } item;
};

struct Library{
   struct SomeItem* someitem;
   struct SomeItem* previousItem;
   struct SomeItem* nextItem;
};

int main(void)
{
   struct Library* biglist;

   biglist->someitem->itemType = MOVIE;

   return 0;
}

(Though this code won't run of course, as I'm not allocating any memory for biglist and someitem!)

回答2:

biglist.someitem.itemType = MOVIE; /* or = MUSIC */

Or, if someitem is a pointer,

biglist.someitem->itemType = MOVIE; /* or = MUSIC */

回答3:

You may initialize the enum in such a way biglist->someitem = MOVIE; but the compiler assigns integer values starting from 0. So: biglist->someitem=MOVIE returns 0 or biglist->someitem=MUSIC return 1

check if it helps any good,

回答4:

struct SomeItem 
{ 
    enum {MOVIE, MUSIC} itemType; 
    union { 
        struct Movie* movie; 
        struct Music* music; 
    } item;
    struct SomeItem *next;
};