Wrapper Façade Pattern E81 CSE 532S: Advanced Multi-Paradigm Software Development Wrapper Façade Pattern Chris Gill, Ryan Cooper, Gene Tien Department of Computer Science and Engineering Washington University, St. Louis cdgill@cse.wustl.edu Title slide

Wrapper Facade Combines related functions/data (OO, generic) pthread_create (thread, attr, start_routine, arg); pthread)_exit (status); pthread_cancel (thread); … thread thread (); thread (function, args); ~thread(); join(); … Contrast with Fascade Combines related functions/data (OO, generic) Used to adapt existing procedural APIs Offers better interfaces Concise, maintainable, portable, cohesive, type safe

Wrapper Façade Intent: Why? • To protect the application programmer from worrying about platform-specific API’s • To simplify use of the API’s (less tedious/error-prone) • To do this while paying only a small performance cost • Other related patterns have different intents Façade Decorator Bridge Adapter Simpifies -- encapsulates, type safety, maintainibility

Wrapper Façade Context: When? When extensibility can be achieved at a suitably low cost (vs. direct use of API) When extensibility is more important than performance improvements Other design patterns can give even more significant flexibility improvements, but the performance costs often increase as well E.g., the Extension Interface pattern

Wrapper Façade Context: Where? ACE_INLINE ACE_thread_t ACE_OS::thr_self (void) { // ACE_OS_TRACE ("ACE_OS::thr_self"); #if defined (ACE_HAS_THREADS) # if defined (ACE_HAS_PTHREADS) // Note, don't use "::" here since the // following call is often a macro. ACE_OSCALL_RETURN (pthread_self (), int, -1); # elif defined (ACE_HAS_STHREADS) ACE_OSCALL_RETURN (::thr_self (), int, -1); # elif defined (ACE_HAS_WTHREADS) return ::GetCurrentThreadId (); # elif defined (ACE_PSOS) // there does not appear to be a way to get // a task's name other than at creation return 0; # elif defined (VXWORKS) return ::taskName (::taskIdSelf ()); # endif /* ACE_HAS_STHREADS */ #else return 1; // Might as well make it the first thread ;-) #endif /* ACE_HAS_THREADS */ } Most commonly used with networking, threading, and other low-level APIs

Wrapper Façade Implementation: How? (from POSA2) Identify cohesive abstractions and relationships among low level APIs E.g., “Every IPC SAP has a handle and every socket is an IPC SAP, but not every IPC SAP is a socket.”

Wrapper Façade Implementation, cont. 2. Cluster cohesive groups of functions into wrapper façade classes and methods 2.1 - Create cohesive classes 2.2 - Coalesce multiple individual functions into a single method (e.g., bind/listen) 2.3 - Automate creation and destruction 2.4 - Select level of indirection 2.5 - Determine where to encapsulate platform-specifics

Wrapper Façade Implementation, cont. 3. Consider allowing selective access to some implementation details, e.g., thread::native_handle_type tnht = t1.native_handle() is a “controlled violation of encapsulation.” 4. Develop error handling (language support for features such as exceptions may matter) 5. Define related helper classes, associated types, etc.

Common Problems and Issues Exceptions Language support Different ways of handling Resource management (e.g., C++ memory leaks) Overhead Loss of functionality Poor cross-language support Performance costs relative to application needs