g_hash_table_lookup_iter

Submitted by Allison (desrt)

Description

It's quite often that you want to perform 'complex' operations on a hashtable.

Things like "lookup and modify if exists, or create otherwise" or "lookup and modify, possibly removing".

GHashTableIter is quite a good way to support this. We already support remove/replace operations on it.

gboolean g_hash_table_lookup_iter       (GHashTable     *table,
                                         constgpointer   key,
                                         GHashTableIter *iter);

gpointer g_hash_table_iter_get_data     (GHashTableIter *iter);

Then we could do stuff like:

  if (g_hash_table_lookup_iter (table, key, &iter))
    {
      mydata = g_hash_table_iter_get_data (&iter);

      ...modify mydata...

      if (mydata->should_be_removed)
        g_hash_table_iter_remove (&iter);
    }

and a similar example with outright replacement.

Now onto the real crack:

One other common pattern that's a bit annoying is "lookup and insert if missing". That one is a bit more complicated to implement because we'd have to support GHashTableIter pointing to non-existent potential future items. I don't really care for that possibility, so we could have instead:

gboolean g_hash_table_insert_or_lookup_iter (GHashTable     *table,
                                             gconstpointer   key,
                                             GCopyFunc       key_copy_func,
                                             gpointer        copy_user_data,
                                             GHashTableIter *iter);

This would either lookup the iter (as above) or, if it didn't exist, copy the key and insert it with NULL data (presumably to be modified later with g_hash_table_iter_replace()).

Edited Oct 17, 2018 by Emmanuele Bassi