如何将 C++ boost posix_time::ptime 的分辨率更改为毫秒？

Question

I'm trying to print a boost::posix_time::ptime in the format of "YYYY-MM-DD hh:mm:ss.fff" where the MM is a 2-digit month and fff a 3-digit millisecond . An example is: 2011- 07 -14 22:38:40. 123 .

I'm using Boost 1.33.1 .

I tried to call the API "ptime::to_simple_string()" but it doesn't meet my need: this method prints the ptime in a format of "YYYY-MMM-DD hh:mm:ss.ffffff" where the "MMM" is a 3-char month name, such as "Jan", and "ffffff" is a 6-digit microsecond. This is NOT what I want.

I then tried to use the time_facet thing:

ptime my_time(second_clock::local_time());
time_facet<ptime, char> * my_time_facet = new time_facet<ptime, char>();
std::cout.imbue(std::locale(std::locale::classic(), my_time_facet));
std::cout << my_time;

But I got an output like "2011-07-14 22:38:40.123000". This is NOT what I want, either.

Looks like the ptime uses the microsecond as the time resolution by default. But I just need a millisecond resolution. After some study I think there could be two methods to solve this problem :

1). Somehow I can change the ptime's time resolution to use millisecond. I've found some enumation and template classes related to the time resolution but I don't know how to use them.

2). Somehow I can change my time facet format to print out just the millisecond. But the official documentation seems to say "%F" and "%f" both use the microsecond resolution.

I tend to prefer the first solution but I need help? How can I get a ptime string in the format I desire?

PS:

1). Though I've tried to search a similar question in this website but I don't find one. If you know it please tell me.

2). The Boost 1.33.1 Date Time documentation(http://www.boost.org/doc/libs/1_33_1/doc/html/date_time.html) is the only reference I've been reading.

---

Update on 07/15/2011:

After some further study, my conclusion is as follows:

1). It is possible to change the time resolution to millisecond so using the "%f" in a time_facet will print out just the milliseconds, instead of the default microseconds. However, you seems to need to define a whole suite of your own classes including your_time_res_traits, your_time_duration, your_time_system_config, your_time_system and your_time. This is definitely too complicated for my problem.

2). Thus I'd take Mark's suggestion(see below) to just strip out the last 3 chars after converting a microsecond-resolution ptime to a string.

Answer 1

The simple, pragmatic solution would be to use %f and always strip the last 3 characters from the result.

Answer 2

//
// microsec_clock
//


  #include "boost/date_time/posix_time/posix_time.hpp"
  #include "boost/date_time/local_time_adjustor.hpp"
  #include "boost/date_time/c_local_time_adjustor.hpp"
  #include <iostream>
////////////////////////////////////////////////////////////////////////////////
void main ()
{
boost::posix_time::ptime my_time(boost::posix_time::microsec_clock::local_time());
boost::date_time::time_facet<boost::posix_time::ptime, char> * my_time_facet = new boost::date_time::time_facet<boost::posix_time::ptime, char>();
std::cout.imbue(std::locale(std::locale::classic(), my_time_facet));
std::cout << my_time;

getchar();
}

Answer 3

This will truncate the fractional seconds.

#include <iostream>
#include <iomanip>
#include <boost/date_time/gregorian/gregorian.hpp>
#include <boost/date_time.hpp>
#include <boost/date_time/posix_time/posix_time.hpp>

int main()
{
  boost::gregorian::date dayte(boost::gregorian::day_clock::local_day());
  boost::posix_time::ptime midnight(dayte);
  boost::posix_time::ptime
     now(boost::posix_time::microsec_clock::local_time());
  // boost::posix_time::time_duration td = now - midnight;
  boost::posix_time::time_duration td(1,2,3,4567899);

  std::stringstream sstream;
  sstream << td.fractional_seconds();

  std::string trunc = std::string(sstream.str()).substr(0,3);

  std::cout << dayte.year() << "-" << dayte.month().as_number()
    << "-" << dayte.day() << " ";
  std::cout << td.hours() << ":" << td.minutes() << ":" << td.seconds()
   << ":" << td.fractional_seconds() << std::endl;

  std::cout << std::endl;
  std::cout << std::setfill('0') << std::setw(2)  << td.hours() << ":";
  std::cout << std::setfill('0') << std::setw(2) << td.minutes() << ":";
  std::cout << std::setfill('0') << std::setw(2) << td.seconds() << ":";
  std::cout << trunc << std::endl;
}

Results

2015-10-27 1:2:7:567899

01:02:07:567