Oooohh, I think I understand now: the way it was is already sufficiently
general.
The writer of the initial code took a different direction.
Instead of defining a union type, he changes the tag of the pointer
depending on what kind of type he needs in the union:
(cpointer-push-tag! e XAnyEvent-tag)
(case (XAnyEvent-type e)
((KeyPress) (cpointer-push-tag! e XKeyPressedEvent-tag))
((KeyRelease) (cpointer-push-tag! e XKeyReleasedEvent-tag))
....
It's pretty smart. Is it the right way to go in general, and should _unions
be avoided?
Laurent
On Fri, Oct 26, 2012 at 6:39 PM, Laurent <laurent.ors...@gmail.com> wrote:
> Hi,
>
> I am trying to convert an FFI cstruct definition into a union.
> More exactly, the previous definition was :
> (define-cstruct _XEvent
> ((type EventType)))
>
> with EventType being defined elsewhere.
> But this was an incomplete definition, and I now want to translate the
> following C union:
> typedef union _XEvent {
> int type; /* must not be changed; first element */
> XAnyEvent xany;
> XKeyEvent xkey;
> // ... the rest for later
> } XEvent;
>
> I searched the collects directory, but I see only a single and too simple
> _union use, so I'm not sure how to use it.
> I guessed the following:
> (define _XEvent
> (_union EventType ; type
> _XAnyEvent ; xany
> _XKeyEvent ; xkey
> ; etc.
> ))
> (define (XEvent-type ev) (union-ref ev 0))
> (define (XEvent-xany ev) (union-ref ev 1))
> (define (XEvent-xkey ev) (union-ref ev 2))
>
> However, now I don't use define-cstruct anymore (it would not work with
> _union, would it?), so I don't have all the -tag, -pointer, etc. bindings,
> which I need.
>
> What should I do ?
>
> Thanks,
> Laurent
>
>
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