How to check that timezone exists with boost locale

Question

I need to parse time from string (%Y-%M-%d %H:%m:%s) according to some timezone.

My first idea was to try boost::date_time, however it looks like its database is outdated and timezone detection algorithm is wrong in general. So I decided to try boost::locale. It has ICU backend, so timezone support should be good. I use the following code:

namespace as = boost::locale::as;

void foo(std::string time, std::string timezone) {
    auto glob = boost::locale::localization_backend_manager::global();
    glob.select("icu"); // select icu backend
    boost::locale::generator gen{glob};

    auto loc = gen.generate(""); // generate locale with boost facets
    auto cal = boost::locale::calendar{loc, timezone};

    boost::locale::date_time dt{cal};

    std::stringstream ss{time};
    ss.imbue(loc);
    std::cout.imbue(loc);

    ss >> as::ftime("%Y-%m-%d %T") >> as::time_zone(timezone) >> dt;
    std::cout << as::time_zone("UTC") << dt << std::endl;
    std::cout << as::time_zone(timezone) << dt << std::endl;
}

This works well, however if I pass some invalid timezone name ("foo"), the library accepts it, no exception is thrown, the time is parsed as if it is UTC time. That's not good for me, I want to detect this case somehow, so that I can notify user that the result will not be what he/she expects.

My first idea was to check cal.get_time_zone(), but it always returns the string that was passed to constructor ("foo" in my case), no matter if it's valid or not.

Next, I tried to extract calendar_facet from the generated locale, like so:

const auto &icu_cal = std::use_facet<boost::locale::calendar_facet>(loc);

so that I can access an internal abstract_calendar class. Unfortunately, this line doesn't compile. The reason is that boost/locale/generator.hpp has a static constant with the same name (calendar_facet) in the same boost::locale namespace. The compiler reports that it can not instantiate std::use_facet. Maybe I can move it to a separate compilation unit and avoid including generator.hpp header there, but it looks like a hack for me. Is it a bug or I'm missing something here?

Is there a straightforward way how to validate timezone name with boost::locale? Do you recommend it in general? Thanks for your help.

Edit: here is a minimal example of code that doesn't compile for me

#include <boost/locale.hpp>
int main() {
  auto my = boost::locale::localization_backend_manager::global();
  my.select("icu");
  boost::locale::generator gen{my};
  std::use_facet<boost::locale::calendar_facet>(gen.generate(""));
  return 0;
}

I compile it like so (on ubuntu 16.04, gcc 5.4):

g++ -std=c++14 -L/usr/lib/x86_64-linux-gnu/ test.cpp -lboost_locale -lboost_date_time

Edit 2: With Sehe's help I managed to get calendar facet from locale and now can I check timezone like this:

int main(int argc, char **argv) {
  auto my = boost::locale::localization_backend_manager::global();
  my.select("icu");
  boost::locale::generator gen{my};
  auto ptr = std::unique_ptr<boost::locale::abstract_calendar>(std::use_facet<class boost::locale::calendar_facet>(gen.generate("")).create_calendar());
  ptr->set_timezone(argv[1]);
  // if ICU backend does not recognize timezone, it sets it to Etc/Unknown
  if (ptr->get_timezone() != argv[1]) {
      std::cout << "bad timezone " << ptr->get_timezone() << std::endl;
  } else {
      std::cout << "good timezone " << ptr->get_timezone() << std::endl;
  }
  return 0;
}

Update: while I managed to make boost locale do what I want on linux, I later faced some weird errors when I ported my code to OS X (it looks like mac doesn't have ICU backend by default...). So, I decided to switch to Howard Hinnant's date library instead. This library is of a high quality, works well on both linux and mac, author is helpful and responsive, so highly recommended.

Answer 1

The fix to the non-compiling sample:

Live On Coliru

#include <boost/locale.hpp>
int main() {
  auto my = boost::locale::localization_backend_manager::global();
  my.select("icu");
  boost::locale::generator gen{my};
  std::use_facet<class boost::locale::calendar_facet>(gen.generate(""));
}

Answer 2

Here is an alternative timezone library that may be easier to use:

#include "tz.h"
#include <iostream>
#include <sstream>

int
main(int argc, char **argv)
{
    try
    {
        auto tz = date::locate_zone(argv[1]);
        std::cout << "good timezone " << tz->name() << std::endl;
        date::local_seconds tp;
        std::istringstream in{"2017-09-08 11:30:15"};
        in >> date::parse("%Y-%m-%d %H:%M:%S", tp);
        auto zt = date::make_zoned(tz, tp);
        std::cout << date::format("%Y-%m-%d %T %Z which is ", zt);
        std::cout << date::format("%Y-%m-%d %T %Z\n", zt.get_sys_time());
    }
    catch (std::exception const& e)
    {
        std::cout << "bad timezone " << e.what() << std::endl;
    }
}

Sample output 1:

good timezone America/New_York
2017-09-08 11:30:15 EDT which is 2017-09-08 15:30:15 UTC

Sample output 2:

bad timezone America/New_Yor not found in timezone database