Apache C++ Standard Library is a complete implementation of
the C++ Standard Library conforming to the ISO/IEC 14882:2003
International Standard for the Programming Language C++.

4.2.1 is a "bugfix" release of the library that is source and
both backward and forward binary compatible with stdcxx 4.2.0.
Programs linked with previous versions of stdcxx 4.x can safely
upgrade to 4.2.1 without needing to be recompiled. New programs
linked with stdcxx 4.2.1 can be deployed to environments with
only stdcxx 4.2.0 installed. Forward compatibility with stdcxx
4.1.x is not guaranteed.