These are not versions of the language, but rather versions of the USL translator, cfront. However many people discuss these as levels of the language as well, since each of the above versions represents additions to the portion of the C++ language which is implemented by the compiler. When ANSI/ISO C++ is finalized, conformance with the ANSI/ISO spec will become more important than conformance with cfront version X.Y, but presently, cfront is acting as a de facto standard to help coordinate the industry (although it leaves certain features of the language unimplemented as well).
*VERY* roughly speaking, these are the major features:
* 2.0 includes multiple/virtual inheritance and pure virtual functions.
* 2.1 includes semi-nested classes and 'delete [] ptr_to_array'.
* 3.0 includes fully-nested classes, templates and 'i++' vs '++i'.
