In MSVC 6.0, the following snippet fails: #include <boost/archive/text_iarchive.hpp> #include <boost/tuple/tuple.hpp> int main() { return 0; } complaining about is_abstract being already defined. The problem lies in the following lines inside boost/serialization/is_abstract.hpp: #ifndef BOOST_TT_IS_ABSTRACT_CLASS_HPP #if defined(__GNUC__) && (34 <= _GNUC__ * 10 + __GNU_MINOR) \ || defined(__MSVC_VER) && (1310 <= __MSVC_VER) \ || defined(__EDG_VERSION__) \ /**/ #include <boost/type_traits/is_abstract.hpp> #else // default to false if not supported namespace boost { template<class T> struct is_abstract { typedef mpl::bool_<false> type; BOOST_STATIC_CONSTANT(bool, value = is_abstract::type::value); }; } // namespace boost #endif #endif // BOOST_TT_IS_ABSTRACT_CLASS_HPP The code is *incorrectly* using __MSVC_VER where it should use _MSVC_VER (just one leading underscore). Thus boost::is_abstract is defined twice, as boost/type_traits.hpp is implicitly included by boost/tupe/tuple.hpp If nobody complains, I can fix this and commit it to the release and main branch. What worries me, though, is that the code above seems flawed to me (apart from the minor typo). What happens if the compiler is not one of those taken care of explicitly in boost/serialization/is_abstract.hpp? Could anyone please test the first snippet in some compiler other than MSVC, ICC for Windows, GCC or an EDG-based compiler? My thesis is that it will fail. Thank you, Joaquín M López Muñoz Telefónica, Investigación y Desarrollo