Format a posix time with just 3 digits in fractional seconds

问题

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

回答1:

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
215a220
>     static const char_type* fractional_seconds_3digits;               // %3
397a403,411
>       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.

回答2:

Here is a way to print the string that is somewhat easier than patching the Boost library (!!). The key idea is to notice that boost::posix_time::to_simple_string() does not print the fractional part if it is precisely zero. So set the fractional part to zero, use to_simple_string(), then manually print the milliseconds yourself.

This leads to the next question: how do you strip the fractional part out of a boost ptime? The easiest way I have found (not necessary the easiest way that exists!) is to go via a time_duration using some absolute reference point. I have chosen 1970-01-01 for tradition's sake, but it doesn't matter what time point you choose (so long as it does not have a fractional seconds part!).

std::string time_with_milli(boost::posix_time::ptime time)
{
    using namespace boost::posix_time;
    static const ptime epoch(boost::gregorian::date(1900, 1, 1));  // Can be any date

    boost::posix_time::time_duration relative_time = time - epoch;
    boost::posix_time::ptime date_time_seconds_only = 
        epoch + boost::posix_time::seconds(relative_time.total_seconds());
    uint64_t milliseconds = relative_time.total_milliseconds() % 1000;

    std::stringstream ss;
    ss << to_simple_string(date_time_seconds_only) << '.' 
       << std::setfill('0') << std::setw(3) << milliseconds;
    return ss.str();
}

This works equally well with to_simple_string(), to_iso_string() and to_iso_extended_string(), even though the documentation for them claims that only to_simple_string() omits the fractional part if it is zero.

来源：https://stackoverflow.com/questions/28136660/format-a-posix-time-with-just-3-digits-in-fractional-seconds

标签

c++

boost

time

datetime-format

time-format