如何为缺少预定义运算符而不扩展命名空间`std`的标准类型定义运算符>>（istream＆，...）？

Question

Apparently it's Undefined Behaviour to add ( almost ) anything to the std namespace.

I'm using C++14 which has no std::chrono::parse() (C++20 only) but I need to deserialise values of type std::chrono::milliseconds (a specialisation of std::chrono::duration ) from an istream .

Although this works, I can't find any exceptions that allow this to not be UB:

namespace std {
std::istream& operator >>(std::istream & is, std::chrono::milliseconds & ms) {
    std::string s;
    is >> s;
    ms = std::chrono::milliseconds(std::stoi(s));
    return is;
}
}

Since neither of these two parameter types are my types, I'm not sure how I'd safely define this operator outside of the std namespace.

Note that the operator will be called deep within Boost::program_options so I don't think I can define the operator in my own namespace and then use using my_ns::operator>> because the scope of that using declaration won't extend into the program_options scope.

How I Ended Up Here

As a user of Boost::program_options I have a particular configuration variable that I read from a configuration file that is stored as a std::chrono::milliseconds value:

std::chrono::milliseconds period;
po::options_description config_only_opts;
config_only_opts.add_options()
    ("control.period", po::value<std::chrono::milliseconds>(&period), "Specify the period in milliseconds");

// ...

auto istream = ifstream("config.cfg");
po::variables_map vm;
po::store(po::parse_config_file(istream, config_file_opts, false), vm);

According to the Boost::program_options documentation, it is possible to deserialise values from the config file for types that have an operator >> (istream &, ...) function defined.

Without the extention to std mentioned above, I end up with this kind of compiler error:

/home/david/opt/boost-1.69/include/boost/lexical_cast/detail/converter_lexical.hpp: In instantiation of ‘struct boost::detail::deduce_target_char_impl<boost::detail::deduce_character_type_later<std::chrono::duration<long int, std::ratio<1, 1000> > > >’:
/home/david/opt/boost-1.69/include/boost/lexical_cast/detail/converter_lexical.hpp:270:89:   required from ‘struct boost::detail::deduce_target_char<std::chrono::duration<long int, std::ratio<1, 1000> > >’
/home/david/opt/boost-1.69/include/boost/lexical_cast/detail/converter_lexical.hpp:407:92:   required from ‘struct boost::detail::lexical_cast_stream_traits<std::__cxx11::basic_string<char>, std::chrono::duration<long int, std::ratio<1, 1000> > >’
/home/david/opt/boost-1.69/include/boost/lexical_cast/detail/converter_lexical.hpp:468:15:   required from ‘struct boost::detail::lexical_converter_impl<std::chrono::duration<long int, std::ratio<1, 1000> >, std::__cxx11::basic_string<char> >’
/home/david/opt/boost-1.69/include/boost/lexical_cast/try_lexical_convert.hpp:201:44:   required from ‘bool boost::conversion::detail::try_lexical_convert(const Source&, Target&) [with Target = std::chrono::duration<long int, std::ratio<1, 1000> >; Source = std::__cxx11::basic_string<char>]’
/home/david/opt/boost-1.69/include/boost/lexical_cast.hpp:41:60:   required from ‘Target boost::lexical_cast(const Source&) [with Target = std::chrono::duration<long int, std::ratio<1, 1000> >; Source = std::__cxx11::basic_string<char>]’
/home/david/opt/boost-1.69/include/boost/program_options/detail/value_semantic.hpp:92:36:   required from ‘void boost::program_options::validate(boost::any&, const std::vector<std::__cxx11::basic_string<charT> >&, T*, long int) [with T = std::chrono::duration<long int, std::ratio<1, 1000> >; charT = char]’
/home/david/opt/boost-1.69/include/boost/program_options/detail/value_semantic.hpp:184:21:   required from ‘void boost::program_options::typed_value<T, charT>::xparse(boost::any&, const std::vector<std::__cxx11::basic_string<charT> >&) const [with T = std::chrono::duration<long int, std::ratio<1, 1000> >; charT = char]’
/home/david/myproj/Config.cpp:208:1:   required from here
/home/david/opt/boost-1.69/include/boost/lexical_cast/detail/converter_lexical.hpp:243:13: error: static assertion failed: Target type is neither std::istream`able nor std::wistream`able
             BOOST_STATIC_ASSERT_MSG((result_t::value || boost::has_right_shift<std::basic_istream<wchar_t>, T >::value),
             ^

Answer 1

I would not recommend adding that function even if the standard allowed the operator to be overloaded. It's not a generic way to read a std::chrono::milliseconds from a std::istream . What you have is your application's own way of reading such an object from a std::istream .

I recommend adding a function in your application's own namespace .

namespace MyApp
{
   std::istream& readChronoMilliSeconds(std::istream& is, std::chrono::milliseconds & ms)
   {
      std::string s;
      is >> s;
      ms = std::chrono::milliseconds(std::stoi(s));
      return is;
   }
}

Answer 2

Using fancy types for input parameter is fooling yourself because input parameters generally are nothing else than numbers, strings or lists of them.

What I recommend is to take a proper integer value and convert internally to milliseconds.

int period_in_ms = 0;
...
config_only_opts.add_options()
    ("control.period_in_ms", po::value<int>(&period_in_ms), "Specify the period in milliseconds");
...
auto period = std::chrono::milliseconds(period_in_ms);

You can use the "notify" mechanism in Boost.ProgramOptions to update the value of period in-place but it is not worth the trouble.

If you want to input the parameter in a certain format (eg finish with "ms", as in "100ms") then take a value<std::string> and make the conversion/parsing.