This structure is used to provide the type system with the information required to initialize and destruct (finalize) a parameter's class and instances thereof. The initialized structure is passed to the g_param_type_register_static() The type system will perform a deep copy of this structure, so its memory does not need to be persistent across invocation of g_param_type_register_static().
Size of the instance (object) structure.
Prior to GLib 2.10, it specified the number of pre-allocated (cached) instances to reserve memory for (0 indicates no caching). Since GLib 2.10, it is ignored, since instances are allocated with the [slice allocator][glib-Memory-Slices] now.