AMDG I'm looking at two recent posts: http://comments.gmane.org/gmane.comp.lib.boost.devel/177370 http://comments.gmane.org/gmane.comp.lib.boost.user/37476 Either of these is fine by itself, but putting them both together makes it very easy to cause silent ODR problems. For example in translation unit a.cpp we #include "boost/tuple/tuple_io.hpp" and translation unit b.cpp we forget. Then, along comes boost::any which detects operator<< in translation unit a.cpp, but uses its default in translation unit b.cpp. Ka-Boom. This is not a huge problem at the moment, because it's quite difficult to make the detection bullet-proof, but I'm very concerned because C++0x concepts are supposed to make such overloading easy. In Christ, Steven Watanabe