You can download the code at the Boost vault: http://boost-consulting.com/vault/index.php?&direction=0&order=&directory=bitfield test.cpp contains a regression test that you can try on your compiler. Please let me know if it works, as I've only tested this on MinGW GCC 3.4. The bitfield mechanism relies on this assumption: Unions of non-polymorphic, non-derived objects, having the exact same underlying data member type, will have the same size as this underlying data member type. I'm no language lawyer, so please let me know if this is a safe and portable assumption. Let me know if things aren't sufficiently self-explanatory, or if things can be improved. -- Emile Cormier emilecormier@mailcan.com