1. "io_service" destructor calls "io_service::service::shutdown_service"; 

2. documentation of "io_service::service::shutdown_service" says that it should destroy all user defined handler objects.

Is it true that user defined handler objects may access to (e.g. post a handler) that "io_service" being destroyed and therefore has not exist already?

Could You please also clarify me what does "user defined handler object" really mean?