Re: embedded list v2

Hi,

On Sunday, September 16, 2012 04:23:14 PM Andres Freund wrote:
> What do you think about something like:
>
> typedef struct dlist_iter
> {
> /*
> * Use a union with equivalent storage as dlist_node to make it possible
> to * initialize the struct inside a macro without multiple evaluation. */
> union {
> struct {
> dlist_node *cur;
> dlist_node *end;
> };
> dlist_node init;
> };
> } dlist_iter;
>
> typedef struct dlist_mutable_iter
> {
> union {
> struct {
> dlist_node *cur;
> dlist_node *end;
> };
> dlist_node init;
> };
> dlist_node *next;
> } dlist_mutable_iter;
>
> #define dlist_iter_foreach(iter, ptr) \
> for (iter.init = (ptr)->head; iter.cur != iter.end; \
> iter.cur = iter.cur->next)
>
> #define dlist_iter_foreach_modify(iter, ptr)
\
> for (iter.init = (ptr)->head, iter.next = iter.cur->next; \
> iter.cur != iter.end \
> iter.cur = iter.next, iter.next = iter.cur->next)
>
> With that and some trivial changes *all* multiple evaluation possibilities
> are gone.
>
> (_iter_ in there would go, thats just so I can have both in the same file
> for now).

I am thinking whether a macro like:

#if __GNUC__ > 4 || (__GNUC__ == 4 && __GNUC_MINOR__ >= 6)
#define assert_compatible_types(a, b) _Static_assert( \
__builtin_types_compatible_p(a, __typeof__ (b) ), \
"variable `" #b "` is not compatible to type `" #a "`" )
#else
#define assert_compatible_types(a, b) (void)0
#endif

used like:

#define dlist_iter_foreach(iter, ptr) \
assert_compatible_types(dlist_iter, iter); \
for (iter.init = (ptr)->head; iter.cur != iter.end; \
iter.cur = iter.cur->next)

would be useful.

If you use the wrong type you get an error like:

error: static assertion failed: "variable `iter` is not compatible to type
`dlist_iter`"

Do people think this is something worthwile for some of the macros in pg? At
times the compiler errors that get generated in larger macros can be a bit
confusing and something like that would make it easier to see the originating
error.

I found __builtin_types_compatible while perusing the gcc docs to find whether
there is something like __builtin_constant_p for checking the pureness of an
expression ;)

Andres
--
Andres Freund http://www.2ndQuadrant.com/
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