Does Boost.Variant require certain warnings to be disabled? The following program generates a warning for me with VC9: COMPILER INPUT: #include <boost/variant.hpp> #include <iostream> struct vis : public boost::static_visitor<> { void operator()( int & ) const { std::cout << "int" << std::endl; } void operator()( std::string & ) const { std::cout << "string" << std::endl; } }; int main( int, char * ) { boost::variant< int, std::string > v; boost::apply_visitor( vis(), v ); return 0; } COMPILER OUTPUT: 1>Compiling... 1>variant_basic.cpp 1>J:\work\ext\boost_ext\boost/variant/variant.hpp(894) : warning C4512: 'boost::detail::variant::invoke_visitor<Visitor>' : assignment operator could not be generated 1> with 1> [ 1> Visitor=const vis 1> ] 1> J:\work\ext\boost_ext\boost/variant/variant.hpp(1771) : see reference to class template instantiation 'boost::detail::variant::invoke_visitor<Visitor>' being compiled 1> with 1> [ 1> Visitor=const vis 1> ] 1> J:\work\ext\boost_ext\boost/variant/detail/apply_visitor_unary.hpp(72) : see reference to function template instantiation 'void boost::variant<T0_,T1>::apply_visitor<const Visitor>(Visitor &)' being compiled 1> with 1> [ 1> T0_=int, 1> T1=std::string, 1> Visitor=vis 1> ] 1> ..\indep\variant_basic.cpp(11) : see reference to function template instantiation 'void boost::apply_visitor<vis,boost::variant<T0_,T1>>(const Visitor &,Visitable &)' being compiled 1> with 1> [ 1> T0_=int, 1> T1=std::string, 1> Visitor=vis, 1> Visitable=boost::variant<int,std::string> 1> ]