From: Peter Jay Salzman <[EMAIL PROTECTED]>
>
> hi there,
>
> using sdl i draw a number of circles (electric charges) and would like
> to know when the user clicks on a charge. a basic charge is a struct:
>
> typedef struct {
> SDL_Surface *img;
> int magnitude;
> SDL_Rect rect;
> } ChargeStruct;
>
> and i have a double linked list of them:
>
> GList *ChargeList = NULL;
> ChargeStruct puck;
> ...
> ChargeList = g_list_append (ChargeList, &puck);
>
> i'd like to take the mouse coordinates on a button click and check to
> see if the click happened on a charge. if it finds a clicked charge, it
> returns a pointer to the ChargeStruct belonging to the clicked charge:
>
>
> ChargeStruct *ChargeClicked(GList *ChgList, SDL_Event event)
> {
> GList *ptr;
> int x, y;
>
> while (ptr != NULL) {
> ptr = g_list_next(ChgList);
> x = ptr->data->rect.x;
> y = ptr->data->rect.y;
> if (event.button.x > x - 10 && event.button.x < x + 10) {
> printf("click\n");
> }
> }
> return(ptr->data);
> }
>
> i don't have any experience with GList's, so i'm grasping at straws.
>
> ptr->data should be a pointer to a ChargeStruct. so ptr->data->rect
> should be the rect element of a ChargeStruct which is pointed to by
> data.
>
> but this function gives the compile errors:
>
> functions.c: In function `ChargeClicked':
> functions.c:21: warning: dereferencing `void *' pointer
> functions.c:21: request for member `rect' in something not a structure or union
> functions.c:22: warning: dereferencing `void *' pointer
> functions.c:22: request for member `rect' in something not a structure or union
>
> i feel like i'm losing my grip on what a pointer is. the code makes
> sense to me, but obviously i'm mistaken about something.
>
> can someone help me out? how do i access rect.x and rect.y of the
> ChargeStruct pointed to by ptr->data?
>
> thanks!
> pete
This appears to be more of a C typecasting issue than anything else.
What you need is ((ChargeStruct *) ptr->data)->rect, so that
C knows what kind of pointer ptr->data is. GList stores things
as void*, so that you can portably store any kind of pointer there.
You could also do:
ChargeStruct *data;
ptr = g_list_next(ChgList); // Maybe should be g_list_next(ptr) instead?
data = (ChargeStruct*) ptr->data;
and just use "data" in the rest of the code, so things look cleaner.
Ron Steinke
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