Convert ros time to hours-minutes-seconds string in system timezone in C++?

Question

I want to convert ros time stamps out of message headers to std::string in a form like YYYY MM DD HH MM SS.SSS or similar. Optionally not including the year or month or day would be nice.

An example showing converting the stamp to a generic unix time data structure then passing that into a function that can convert from that to a string using a format string would be very useful to have (ros time to time_t to strftime would be nice), also something showing modern C++ and std::chrono.

Ideally a minimum of dependencies (like boost) or blocks of conversion code to cut and paste would be required.

Conversion from ROS timestamp to ISO-8601 time format outputs UTC time using boost and doesn't show any other control over the output format so isn't sufficient.

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Originally posted by lucasw on ROS Answers with karma: 8729 on 2019-12-26

Post score: 3

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Answer 1

Converting to local timezone, not using boost (and the seconds needs to be capital 'S' for strftime):

   std::string stampToString(const ros::Time& stamp, const std::string format="%Y-%m-%d %H:%M:%S")
   {
      const int output_size = 100;
      char output[output_size];
      std::time_t raw_time = static_cast<time_t>(stamp.sec);
      struct tm* timeinfo = localtime(&raw_time);
      std::strftime(output, output_size, format.c_str(), timeinfo);
      std::stringstream ss; 
      ss << std::setw(9) << std::setfill('0') << stamp.nsec;  
      const size_t fractional_second_digits = 4;
      return std::string(output) + "." + ss.str().substr(0, fractional_second_digits);
    }

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Originally posted by lucasw with karma: 8729 on 2019-12-28

This answer was NOT ACCEPTED on the original site

Post score: 3

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Original comments

Comment by rjosodtssp on 2020-11-18:
the best one

Answer 2

The way you control the output format of a boost::posix_time object is by constructing a facet with the format flags that you want and apply the facet to a std::stringstream used to generate the string. It will be clearer with an example:

#include <iostream>
#include <locale>
#include <string>
#include <boost/date_time/posix_time/posix_time.hpp>
#include <boost/date_time/posix_time/posix_time_io.hpp>
#include <ros/ros.h>
...
auto current_time = ros::Time::now().toBoost(); //You asked for modern C++, so... there :P
std::stringstream ss;
boost::posix_time::time_facet facet("%Y %m %d %H %M %s");
ss.imbue(std::locale(std::locale::classic(), &facet));
ss << current_time;
ROS_INFO("Current time: %s", ss.str().c_str());

I hope you find this helpful. (Did this on-the-fly, so this code might not compile, but should put you on the right track)

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Originally posted by Martin Peris with karma: 5625 on 2019-12-26

This answer was NOT ACCEPTED on the original site

Post score: 3

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Original comments

Comment by lucasw on 2019-12-28:
It compiles but crashes when the locale destructor tries to free facet:

#3  0x00007ffff5cf390a in malloc_printerr (
    str=str@entry=0x7ffff5e1b7a8 "munmap_chunk(): invalid pointer") at malloc.c:5350  
#4  0x00007ffff5cfaecc in munmap_chunk (p=0x7fffffffb040) at malloc.c:2846
#5  __GI___libc_free (mem=0x7fffffffb050) at malloc.c:3117
#6  0x00007ffff6310834 in std::locale::_Impl::~_Impl() ()
   from /usr/lib/x86_64-linux-gnu/libstdc++.so.6
#7  0x00007ffff63109e4 in std::locale::~locale() () from /usr/lib/x86_64-linux-gnu/libstdc++.so.6

so this instead:

auto facet = new boost::posix_time::time_facet("%Y %m %d %H %M %s");
ss.imbue(std::locale(std::locale::classic(), facet));

(so it would be better to create facet once and reuse it). https://stackoverflow.com/questions/17779660/who-is-responsible-for-deleting-the-facet

Comment by lucasw on 2019-12-28:
std::locale::classic and std::cout.getloc() are giving me UTC time, if that could be made into local time I'd prefer this answer.