Hello ! The following code compiles correctly and does what I want: #include <iostream> #include <string> #include <boost/variant.hpp> #include <gmpxx.h> using namespace std; typedef boost::variant<int,string> var_t; // COMMENT //typedef boost::variant<mpz_class,mpq_class> var_t; // UNCOMMENT class add : public boost::static_visitor<var_t> { public: template<typename T> var_t operator()(const T &op1, const T &op2) const { return(op1+op2); } template<typename T1, typename T2> var_t operator()(const T1 &op1, const T2 &op2) const { throw; // just for demonstration } }; int main() { var_t v0(2); var_t v1(3); var_t r0 = apply_visitor(add(),v0,v1); cout << r0 << endl; // "5" var_t v10("Hello, "); var_t v11("World !"); var_t r10 = apply_visitor(add(),v10,v11); cout << r10 << endl; // "Hello, World !" var_t error= apply_visitor(add(),v0,v10); // try to add "2" and "Hello, " cout << error << endl; // never executed } However, if I change to the "gmp" Variant typedef indicated above, compilation fails with an error in the boost::variant sources. I tried to extract the core error message: [...] /usr/include/boost/variant/variant.hpp:1293: error: call of overloaded ‘initialize(void*, const char [8])’ is ambiguous /usr/include/boost/variant/detail/initializer.hpp:89: note: candidates are: static int boost::detail::variant::make_initializer_node::apply<BaseIndexPair, Iterator>::initializer_node::initialize(void*, typename boost::call_traits<typename boost::unwrap_recursive<typename boost::mpl::deref<Iterator>::type>::type>::param_type) [with BaseIndexPair = boost::mpl::pair<boost::detail::variant::make_initializer_node::apply<boost: :mpl::pair<boost::detail::variant::initializer_root, mpl_::int_<0> >, boost::mpl::l_iter<boost::mpl::list2<__gmp_expr<__mpz_struct [1], __mpz_struct [1]>, __gmp_expr<__mpq_struct [1], __mpq_struct [1]> > >
::initializer_node, mpl_::int_<1> >, Iterator = boost::mpl::l_iter<boost::mpl::list1<__gmp_expr<__mpq_struct [1], __mpq_struct [1]> > >] <near match> /usr/include/boost/variant/detail/initializer.hpp:89: note: static int boost::detail::variant::make_initializer_node::apply<BaseIndexPair, Iterator>::initializer_node::initialize(void*, typename boost::call_traits<typename boost::unwrap_recursive<typename boost::mpl::deref<Iterator>::type>::type>::param_type) [with BaseIndexPair = boost::mpl::pair<boost::detail::variant::initializer_root, mpl_::int_<0> >, Iterator = boost::mpl::l_iter<boost::mpl::list2<__gmp_expr<__mpz_struct [1], __mpz_struct [1]>, __gmp_expr<__mpq_struct [1], __mpq_struct [1]> > >] <near match> [...]
After having a look at the gmp documentation, especially the "12.6 C++ Interface Limitations" chapter, I think this has something to do with the various conversion functions between the gmp classes and the way how the gmp classes are implemented. Is there any (easy?) way to make boost::variant and gmp compatible ? Regards, Sven -- Sven Heithecker GPG Fingerprint: 527F 06C2 BF51 96CE DFF7 CC89 1A6B DFD5 BEC2 02A6