When just: #include <boost/limits.hpp> is just preprocessed, the output contains no definition of CHAR_BIT; yet, it's used (although within a #if..#endif) within limits.hpp. Limits.hpp does contain: #ifdef BOOST_NO_LIMITS # include <boost/detail/limits.hpp> #else # include <limits> #endif which makes me wonder, since CHAR_BIT is used below this code, whether the authors expected CHAR_BIT to be defined at this point? BTW, CHAR_BIT is defined in system header, limits.h. The reason it hasn't caused a problem so far was explained in another post: http://archives.free.net.ph/message/20070820.174844.ac9e6b8d.en.html