A callback function which is called when the reference count of a class drops to zero.
It may use g_type_class_ref() to prevent the class from being freed. You should not call g_type_class_unref() from a #GTypeClassCacheFunc function to prevent infinite recursion, use g_type_class_unref_uncached() instead.
The functions have to check the class id passed in to figure whether they actually want to cache the class of this type, since all classes are routed through the same #GTypeClassCacheFunc chain.
Pointer(Void)? -> Bool