Different methods to use GSList as value of a GHashTable

Platform
8

A general suggestion: when you have this kind of problems is much cleaner to wrap the GLib code with your own specialized functions and hide the implementation as much as you can, for example (untested):

// country.h: public interface

typedef struct _Country Country;

Country *   country_new         (void);
void        country_town_add    (Country *      country,
                                 const gchar *  state,
                                 const gchar *  town);
...


// country.c: implementation

struct _Country {
    GHashTable *states;
};

Country *
country_new(void)
{
    Country *coutry = g_new(Country, 1);
    country->states = g_hash_table_new(g_str_hash, g_str_equal);
    return country;
}

void
country_town_add(Country *country, const gchar *state, const gchar *town)
{
    GSList *towns = g_hash_table_lookup(country->states, state);
    towns = g_slist_append(towns, town);
    g_hash_table_insert(country->states, state, towns);
}

This has the huge advantage you can reimplement country.c in whatever way you want, e.g. by using @pwithnall idea, without having to touch the code above it.