I ran into a bizarre problem trying to build a variant from an MPL vector. The compilation of make_variant_over seems to fail when given any vector which a) uses all its template arguments, and b) contains *only* user-defined types; ones with at least one built-in type work fine! The problem occurs at least with gcc 3.3.5 and gcc 3.4.4 20050203 prerelease, on Debian unstable/x86. The following code demonstrates the problem: //////////////////////////////////////////////////////////////////////// #include <boost/variant.hpp> #include <boost/mpl/vector.hpp> #include <cstdio> // a template for generating dummy types template <int i> struct d { }; typedef boost::mpl::vector < d<0>, d<1>, d<2>, d<3>, d<4>, d<5>, d<6>, d<7>, d<8>, d<9> > vector; boost::make_variant_over<vector>::type v; //////////////////////////////////////////////////////////////////////// The same thing happens with vector10 and others, but lists do seem to work correctly, providing a work-around. Swapping any built-in type for any of the d<>'s makes the code above to compile cleanly. I can post the complete compiler error output if you like, but there's rather much of it :p -Johannes