ViewKit is a C++ library and because of the implementation of the C++ language, is dependent on the specific version of the C++ compiler you are using. If you use libraries built with one C++ compiler, and you try to link those C++ libraries with code or libraries generated with another compiler, you will get link errors at best, and in the worst case, puzzling runtime errors. In some cases, even "minor" releases in the C++ compiler can create incompatibilities (e.g., gcc 2.95 and gcc 2.96 are NOT compatible!) ICS provides ViewKit for a wide variety of C++ compiler releases.