提升程序选项对价值

我观察到 STL 类的 boost::program_options::value 函数的奇怪行为。我经常需要成对地为程序提供参数，例如带有短标签的文件名，但boost::program_options::value函数似乎不适用于std::pair，而它确实适用于我自己定义的任何类。请考虑以下代码：

#include <string>
#include <utility>
#include <boost/program_options.hpp>
using namespace std;
namespace po = boost::program_options;
class sspair: public pair<string,string> { };
typedef pair<string,string> mypair;
// typedef sspair mypair;
istream& operator>>(istream& in, mypair& ss) {
  string s;
  in >> s;
  const size_t sep = s.find(':');
  if (sep==string::npos) {
    ss.first = string();
    ss.second = s;
  } else {
    ss.first  = s.substr(0,sep);
    ss.second = s.substr(sep+1);
  }
  return in;
}
int main(int argc, char **argv)
{
  mypair a;
  try {
    po::options_description all_opt("Options");
    all_opt.add_options()
      ("arg,a", po::value<mypair>(&a),"colon separated pair")
    ;
    po::variables_map vm;
    po::store(po::parse_command_line(argc, argv, all_opt), vm);
    po::notify(vm);
  } catch(exception& e) {
    cerr << e.what() << endl;
    exit(1);
  }
  cout << "a = (" << a.first << ", " << a.second << ")" << endl;
  return 0;
}

有了typedef sspair mypair我得到了预期的行为。

$ ./test 
a = (, )
$ ./test -a b:c
a = (b, c)
$ ./test -a bc
a = (, bc)

但是有了typedef pair<string,string> mypair我得到以下编译错误：

In file included from /usr/include/boost/any.hpp:27:0,
                 from /usr/include/boost/program_options/value_semantic.hpp:12,
                 from /usr/include/boost/program_options/options_description.hpp:13,
                 from /usr/include/boost/program_options.hpp:15,
                 from test.cc:4:
/usr/include/boost/lexical_cast.hpp: In instantiation of ‘struct boost::detail::deduce_target_char_impl<boost::detail::deduce_character_type_later<std::pair<std::basic_string<char>, std::basic_string<char> > > >’:
/usr/include/boost/lexical_cast.hpp:415:89:   required from ‘struct boost::detail::deduce_target_char<std::pair<std::basic_string<char>, std::basic_string<char> > >’
/usr/include/boost/lexical_cast.hpp:674:92:   required from ‘struct boost::detail::lexical_cast_stream_traits<std::basic_string<char>, std::pair<std::basic_string<char>, std::basic_string<char> > >’
/usr/include/boost/lexical_cast.hpp:2363:19:   required from ‘static Target boost::detail::lexical_cast_do_cast<Target, Source>::lexical_cast_impl(const Source&) [with Target = std::pair<std::basic_string<char>, std::basic_string<char> >; Source = std::basic_string<char>]’
/usr/include/boost/lexical_cast.hpp:2543:50:   required from ‘Target boost::lexical_cast(const Source&) [with Target = std::pair<std::basic_string<char>, std::basic_string<char> >; Source = std::basic_string<char>]’
/usr/include/boost/program_options/detail/value_semantic.hpp:89:38:   required from ‘void boost::program_options::validate(boost::any&, const std::vector<std::basic_string<charT> >&, T*, long int) [with T = std::pair<std::basic_string<char>, std::basic_string<char> >; charT = char]’
/usr/include/boost/program_options/detail/value_semantic.hpp:170:55:   required from ‘void boost::program_options::typed_value<T, charT>::xparse(boost::any&, const std::vector<std::basic_string<charT> >&) const [with T = std::pair<std::basic_string<char>, std::basic_string<char> >; charT = char]’
test.cc:49:1:   required from here
/usr/include/boost/lexical_cast.hpp:388:13: error: invalid application of ‘sizeof’ to incomplete type ‘boost::STATIC_ASSERTION_FAILURE<false>’
             BOOST_STATIC_ASSERT_MSG((result_t::value || boost::has_right_shift<std::basic_istream<wchar_t>, T >::value), 
             ^
make: *** [test] Error 1

我发现如果我尝试使用其他 stl 容器（如 std::array 或 std::tuple，也会发生类似的行为。

有人知道问题出在哪里吗？

编辑：

我阅读这篇文章后，我刚刚发现了导致此问题的原因。 显然，流运算符仅在定义类（po：：value 函数的模板参数）的命名空间中查找。所以，通过编辑

namespace std {
  istream& operator>>(istream& in, mypair& ss) { ... }
}

pair<string,string>类直接工作。

现在，在 std 命名空间中定义运算符有什么缺点吗？我听说这本身不符合标准。