OFFSETOF_VAR macro to simplify hashmap iterators

While we cannot rely on a `__typeof__' operator being portable
to use with `offsetof'; we can calculate the pointer offset
using an existing pointer and the address of a member using
pointer arithmetic for compilers without `__typeof__'.

This allows us to simplify usage of hashmap iterator macros
by not having to specify a type when a pointer of that type
is already given.

In the future, list iterator macros (e.g. list_for_each_entry)
may also be implemented using OFFSETOF_VAR to save hackers the
trouble of using container_of/list_entry macros and without
relying on non-portable `__typeof__'.

v3: use `__typeof__' to avoid clang warnings

Signed-off-by: Eric Wong <e@80x24.org>
Reviewed-by: Derrick Stolee <stolee@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

1 files changed, 2 insertions, 6 deletions

diff --git a/revision.c b/revision.c
index 8a5f866..5abd4a1 100644
--- a/revision.c
+++ b/revision.c
@@ -129,9 +129,7 @@ static void paths_and_oids_clear(struct hashmap *map)
 	struct hashmap_iter iter;
 	struct path_and_oids_entry *entry;
 
-	hashmap_for_each_entry(map, &iter, entry,
-				struct path_and_oids_entry,
-				ent /* member name */) {
+	hashmap_for_each_entry(map, &iter, entry, ent /* member name */) {
 		oidset_clear(&entry->trees);
 		free(entry->path);
 	}
@@ -243,9 +241,7 @@ void mark_trees_uninteresting_sparse(struct repository *r,
 		add_children_by_path(r, tree, &map);
 	}
 
-	hashmap_for_each_entry(&map, &map_iter, entry,
-				struct path_and_oids_entry,
-				ent /* member name */)
+	hashmap_for_each_entry(&map, &map_iter, entry, ent /* member name */)
 		mark_trees_uninteresting_sparse(r, &entry->trees);
 
 	paths_and_oids_clear(&map);