boost :: variant和vector中元素的打印方法

 std::vector< boost::variant<std::string, int> > vec;
 std::string s1("abacus");
 int i1 = 42;
 vec.push_back(s1);
 vec.push_back(i1);
 std::cout << vec.at(0).size() << "n";

main.cpp:68: error: ‘class boost::variant<std::basic_string<char, std::char_traits<char>, std::allocator<char> >, int, boost::detail::variant::void_, boost::detail::variant::void_, boost::detail::variant::void_, boost::detail::variant::void_, boost::detail::variant::void_, boost::detail::variant::void_, boost::detail::variant::void_, boost::detail::variant::void_, boost::detail::variant::void_, boost::detail::variant::void_, boost::detail::variant::void_, boost::detail::variant::void_, boost::detail::variant::void_, boost::detail::variant::void_, boost::detail::variant::void_, boost::detail::variant::void_, boost::detail::variant::void_, boost::detail::variant::void_>’ has no member named ‘size’
make: *** [main.o] Error 1

std::cout << vec.at(0) << "n";