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2.7.3 SOM

   OpenDoc uses the System Object Model (SOM) initially released in OS/2 2.0 
   in April 1992.  SOM is a proven, multi-platform, and language-neutral 
   object model.  It is compliant with the CORBA 1.2 specification. 

   SOM was originally introduced as part of OS/2 to provide a stable object 
   model for the Workplace Shell.  The problem by introducing an 
   object-oriented user interface into an operating system platform was the 
   release-to-release binary compatibility (RRBC).  That means that parts 
   could be replaced without having side effects to applications using these 
   parts.  The Workplace Shell was implemented as a class library that could 
   be used by many other application developers inheriting the functionality 
   provided by the Workplace Shell.  What would happen if the Workplace Shell 
   would be upgraded?  Using pure C++ would mean that none of these 
   applications would run without recompilation and redistribution.  The 
   Workplace Shell has solved this problem by using SOM. If the Workplace 
   Shell classes change all Workplace Shell derived classes and applications 
   are inheriting the new functions on the fly. It is due to the magic of SOM 
   providing release-to-release binary compatibility (RRBC). 

   A component technology such as OpenDoc is facing exactly the same 
   problems.  The reason is that there is no binary consistence of changed 
   objects and classes.  That means, if a C++ class is updated, all classes 
   using the changed class or being derived from it must be recompiled. 
   Because of the different object layouts produced by different compilers, 
   it is also not possible to link object codes produced with a compiler A 
   with object codes produced by a compiler B.  A C++ programmer, for 
   example, cannot easily use classes developed in Smalltalk, nor can a 
   Smalltalk programmer make effective use of C++ classes.  Object-oriented 
   language and toolkit boundaries become barriers to interoperability.  A 
   component model has to solve this problem, which it can do this in 
   different ways.  One way to avoid this problem is to not allow inheritance 
   at all.  That is the way COM (Common Object Model), the underlaying object 
   model of OLE2, is working. 

   SOM as the underlying object model of OpenDoc is the IBM solution to that 
   problem.  SOM provides a strong state-of-the art object behavior by using 
   the object model of CORBA.  SOM is supporting multiple inheritance, offers 
   strong encapsulation for objects and polymorphism.  One way to solve the 
   RRBC problem for SOM was to make classes itself to living objects, similar 
   to what Smalltalk is doing.  This provides a great flexibility.  Classes 
   can be created, changed, and updated dynamically at run time.  The same 
   implementation of classes can behave in different ways by using 
   metaclasses, that are classes the class objects are derived from. 

   This binary compatibility enables SOM objects to be implemented and used 
   in a language-independent way.  Classes are defined in this 
   CORBA-compliant IDL.  SOM has added some implementation specific 
   constructs such as metafiles.  The SOM Compiler then generates from this 
   IDL definition header files and skeletons for the language the developer 
   prefers to implement the functions in.  This may be C, C++, COBOL, or REXX 
   for example.  Because SOM provides the object behavior as inheritance, 
   classes can be implemented with non object-oriented languages producing 
   fully fledged class libraries. 

   In addition to that, SOM can produce usage bindings for other languages. 
   That means a class implemented in REXX can be derived from a class 
   implemented in C and can be used from a COBOL or Smalltalk program. 

   SOM scales very gracefully from local to distributed objects, utilizing 
   the distributed SOM (DSOM) framework included in the SOMObjects() 
   Developers Toolkit.  With the SOM/DSOM model, the same object model can be 
   used across the entire spectrum of small and large operating systems 
   covering DOS/Windows, OS/2, AIX, OS/400 and MVS. Other platform support 
   for HP-UX, Apple-Mac and SUN is planned for the near future. 

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