Format a posix time with just 3 digits in fraction

What's the equivalent of microsec_clock for milliseconds in the following code?

#include <boost/date_time/posix_time/posix_time.hpp>

int main()
{
    boost::posix_time::ptime date_time = boost::posix_time::microsec_clock::local_time();
    const std::string str_time = boost::posix_time::to_simple_string(date_time);
    std::cout << str_time << std::endl;
    return 0;
}

output: 2015-Jan-25 16:26:14.932738

I need the following output:

output: 2015-Jan-25 16:26:14.932

Indeed, this seems a strange omission from the library.

Then again, it doesn't look that strftime has the goods: http://en.cppreference.com/w/cpp/chrono/c/strftime

So, what would it take to patch the time_facet implementation to get this? Here's the patch against boost 1.57's boost::date_time::time_facet.hpp (applied).

1d0
< 
39a39
>       static const char_type fractional_seconds_3digits[3];              // 3
72a73,76
>   time_formats<CharT>::fractional_seconds_3digits[3] = {'%','3'};
> 
>   template <class CharT>
>   const typename time_formats<CharT>::char_type
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>     static const char_type* fractional_seconds_3digits;               // %3
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>       if (local_format.find(fractional_seconds_3digits) != string_type::npos) {
>         // replace %3 with nnn
>         if (frac_str.empty()) {
>           frac_str = fractional_seconds_as_3digit_string(time_arg.time_of_day(), false);
>         }
>         boost::algorithm::replace_all(local_format,
>                                       fractional_seconds_3digits,
>                                       frac_str);
>       }
506a521,529
>       if (format.find(fractional_seconds_3digits) != string_type::npos) {
>         // replace %3 with nnn
>         if (frac_str.empty()) {
>           frac_str = fractional_seconds_as_3digit_string(time_dur_arg, false);
>         }
>         boost::algorithm::replace_all(format,
>                                       fractional_seconds_3digits,
>                                       frac_str);
>       }
550a574,592
>     fractional_seconds_as_3digit_string(const time_duration_type& time_arg,
>                                  bool null_when_zero)
>     {
>       typename time_duration_type::fractional_seconds_type frac_sec =
>         time_arg.fractional_seconds();
> 
>       for (auto n = time_arg.num_fractional_digits(); n>3; --n)
>           frac_sec /= 10;
> 
>       if (null_when_zero && (frac_sec == 0)) {
>         return string_type();
>       }
> 
>       //make sure there is no sign
>       return integral_as_string(date_time::absolute_value(frac_sec), 3);
>     }
> 
>     static
>     string_type
599a642,645
> 
>   template <class time_type, class CharT, class OutItrT>
>   const typename time_facet<time_type, CharT, OutItrT>::char_type*
>   time_facet<time_type, CharT, OutItrT>::fractional_seconds_3digits = time_formats<CharT>::fractional_seconds_3digits;

Now you can just use Boost DateTime's time_facet for ptimes:

#include "time_facet.hpp"
#include <boost/date_time/posix_time/posix_time.hpp>

int main()
{
    using namespace boost::posix_time;
    ptime const date_time = microsec_clock::local_time();

    std::cout << date_time << std::endl;

    auto facet = new time_facet("%Y-%b-%d %H:%M:%S.%f %z");
    std::cout.imbue(std::locale(std::cout.getloc(), facet));
    std::cout << date_time << std::endl;

    facet = new time_facet("%Y-%b-%d %H:%M:%S.%3 %z");
    std::cout.imbue(std::locale(std::cout.getloc(), facet));
    std::cout << date_time << std::endl;
}

Which prints

2015-Jan-25 22:32:30.392108
2015-Jan-25 22:32:30.392108
2015-Jan-25 22:32:30.392

Now this is a rough patch, just to show what you'd need to get done iff you were to add this. The relevant improvements would seem to be:

support a format string that allows different numbers of digits in the fractional seconds
use proper rounding (instead of truncating, what happens now)

I hope this sample helps.