ObjectSpace::WeakMap#length

static VALUE
wmap_size(VALUE self)
{
    struct weakmap *w;
    TypedData_Get_Struct(self, struct weakmap, &weakmap_type, w);

    st_index_t n = st_table_size(w->table);

#if SIZEOF_ST_INDEX_T <= SIZEOF_LONG
    return ULONG2NUM(n);
#else
    return ULL2NUM(n);
#endif
}