Dear Boosters, I have some strange problem and don't understand if this is really intended so by the C++ standard or is this a bug in the compiler of MS Visual C++ 2003 & 2005. I have a template class which receives an mpl::vector with possible storage types. This class contains an instance of boost::variant but is intended to wrap it with a little bit different functionality. //Solution A //A body of a _working__ solution: template<class Seq_> class my_union { public: template<class T> my_union(const T& rhs) {...} //... }; //Solution B //not working more general solution //this struct compiles fine template<T> struct const_ref : boost::add_const<typename boost::add_reference<T>::type> {}; template<class Seq_> class my_union { public: template<class T> my_union(typename const_ref<T>::type rhs) {...} //... }; now call of the class: //assume my_pair is defined earlier //assume classes A,B,C,D are defined upon typedef my_pair<0, A> t1; typedef my_pair<1, B> t2; typedef my_pair<2, C> t3; typedef my_pair<3, A> t4; typedef mpl::vector<t1,t2,t3,t4>::type types_vector; typedef my_union<types_vector> union_t; t1 t(A()); union_t u(t); //error! with Solution B, but fine with Solution A in the my_union I get an error stating: error C2664: 'my_union<Seq_>::my_union(const my_union<Seq_> &)' : cannot convert parameter 1 from 't1' to 'const my_union<Seq_> &' with [ Seq_=types_vector ] Reason: cannot convert from 't1' to 'const my_union<Seq_>' with [ Seq_=types_vector ] No user-defined-conversion operator available that can perform this conversion, or the operator cannot be called Why does Solution A work fine and Solution B does not? Is this really intended by a standard? With Kind Regards, Ovanes Markarian