Relationship of object/object_id

    object
    +-----------------------------------------------+
    |space_id                                       |       typedef object_id<protocol_ids, limit_order_object_type,limit_order_object>;
    |type_id                                        |       limit_order_id_type
    |id                                   <---------|------>+-----------------------------------+
    |    (object_id_type)                           |       |space_id = protocol_ids            |
    +-----------------------------------------------+       |type_id = limit_order_object_type  |
                            ^                               |instance                           |
                            |                               +-----------------------------------+
                            |
                            |
    abstract_object
    +-----------------------------------------------+
    |pack()                                         |
    |hash()                                         |
    |                                               |
    +-----------------------------------------------+
                            ^
                            |
                            |
    limit_order_object      |
    +-----------------------------------------------+
    |space_id = protocol_ids                        |
    |type_id = limit_order_object_type              |
    +-----------------------------------------------+