How to check that timezone exists with boost locale - c++

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My first idea was to try boost::date_time, however it looks like its database is outdated and timezone detection algorithm is wrong in general.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.

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(""));
}

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

Related

std::time_point from and to std::string

Am trying to replace some boost::gregorian code using c++20 std::chrono, hoping to remove the boost build depedency. Code is reading and writing to json (using nlohmann) so ability to convert dates to and from std::string is critical.
Using g++ 9.3.0 on Ubuntu 20.04. 2 compile-time erorrs, one on std::chrono::parse() and the second on std::put_time()
For error A on std::chrono::parse(), I see here that calendar support (P0355R7), that includes chrono::parse, is not yet available in gcc libstdc++. Anyone know if this is correct or have a link to an ETA for this? or is there something wrong with how I'm calling parse()?
For error B for std::put_time(): since std:put_time() is documented as c++11 feel like I'm missing something silly here. Also find it strange needing to covert through c's time_t and tm. Is there a better way to convert std::chrono::time_point directly to std::string without resorting to c?
#include <chrono>
#include <string>
#include <sstream>
#include <iostream>
int main(int argc, char *argv[]) {
std::chrono::system_clock::time_point myDate;
//Create time point from string
//Ref: https://en.cppreference.com/w/cpp/chrono/parse
std::stringstream ss;
ss << "2020-05-24";
ss >> std::chrono::parse("%Y-%m-%e", myDate); //error A: ‘parse’ is not a member of ‘std::chrono’
//Write time point to string
//https://en.cppreference.com/w/cpp/io/manip/put_time
//http://cgi.cse.unsw.edu.au/~cs6771/cppreference/en/cpp/chrono/time_point.html
std::string dateString;
std::time_t dateTime = std::chrono::system_clock::to_time_t(myDate);
std::tm tm = *std::localtime(&dateTime);
dateString = std::put_time(&tm, "%Y-%m-%e"); //error B: ‘put_time’ is not a member of ‘std’
//Write out
std::cout << "date: " << dateString << "\n";
return 0;
}
C++20 <chrono> is still under construction for gcc. I've seen no public ETA's for it.
Your syntax for std::chrono::parse looks correct. If you're willing to use a free, open-source, header-only preview of C++20 <chrono> then you can get it to work by adding #include "date/date.h" and using date::parse instead.
Note that the resulting myDate will be 2020-05-24 00:00:00 UTC.
std::put_time lives in the header <iomanip> and is a manipulator. After adding that header and <iostream> you would use it like this:
std::cout << "date: " << std::put_time(&tm, "%Y-%m-%e") << '\n';
If you need the output in a std::string, you will have to stream the manipulator to a std::stringstream first.
C++20 <chrono> will provide an alternative to the C API for formatting:
std::cout << "date: " << std::format("{%Y-%m-%e}", myDate) << '\n';
The preview library also provides this with a slightly altered format string:
std::cout << "date: " << date::format("%Y-%m-%e", myDate) << '\n';

How to "add" user-defined format flags to basic_ostream?

I want to provide a stream operator to output std::chrono::time_point as GMT date, I currently have the following (simplified for ostream only):
using datetime_t = std::chrono::system_clock::time_point;
std::ostream& operator<<(std::ostream &out, datetime_t dt) {
auto time = datetime_t::clock::to_time_t(dt);
auto under_sec =
std::chrono::duration_cast<std::chrono::milliseconds>(
dt.time_since_epoch() % std::chrono::seconds{1});
return out << std::put_time(std::gmtime(&time), "%Y-%m-%dT:%H:%M:%S")
<< "." << std::setfill('0') << std::setw(3) << under_sec.count();
}
Usage:
auto time = datetime_t::clock::now();
std::cout << time;
This works, but it forces the user to:
use the hard-coded format;
output milliseconds.
I would like to provide custom stream manipulator that would allow the user to modify both of these, e.g. for the second (assuming a namespace nm containing the manipulator):
std::cout << nm::us << time;
...that would print up to microseconds.
I already know how to create stream manipulators, e.g.:
namespace nm {
std::ios_base& us(std::ios_base &) { /* ... */ }
}
...but I don't know how to "store" the required information for use in the output operator.
Is there a simply way to "store" information in a stream (user-defined format flags?) to use in a later stream operation? Or another way to obtain slightly equivalent behavior?
As you've already discovered in your comments, yes, streams have, iword and pword storage. Not the easiest thing in the world to work with (designed decades ok), but serviceable.
Another option would be to use an already coded library for this such as Howard Hinnant's free, open source, datetime library:
#include "date.h"
#include <iostream>
int
main()
{
using namespace date;
using namespace std::chrono;
std::cout << format("%FT:%T", floor<microseconds>(system_clock::now())) << '\n';
}
With this library the precision of the output is controlled by adjusting the precision of the input (i.e. with time_point_cast or floor).
Sample output:
2017-07-10T:11:46:59.354321

get current time in specific timezone using c++

My code should work on linux & windlows as well.
I want to get current time in YYYY-MM-DD HH24:MI:SS. Default timezone is UTC+08, where as my system can be located in any timezone.
It would be great help if you can help me with c++ code (I do not have c++11 , 14 compiler)
I saw one solution - use time to get the current time in UTC and then manipulate the TZ environment variable to your destination timezone. Then use localtime_r to convert to that timezone's local time.
But not sure how to achieve this with c++ that will work for Windows and linux.
I suggest looking into the boost library boost/date_time/posix_time/posix_time.hpp.
From there you can then simply get the current local time like so:
boost::posix_time::ptime curr_time = boost::posix_time::microsec_clock::local_time();
And it has methods to turn it into a string as required:
std::string curr_time_str = to_simple_string(curr_time);
And back to the ptime object:
curr_time = boost::posix_time::time_from_string(curr_time_str);
etc.
http://www.boost.org/doc/libs/1_61_0/doc/html/date_time/posix_time.html
Should work on most platforms:
int main(int argc, const char * argv[])
{
time_t ts = 0;
struct tm t;
char buf[16];
::localtime_r(&ts, &t);
::strftime(buf, sizeof(buf), "%z", &t);
std::cout << "Current timezone: " << buf << std::endl;
::strftime(buf, sizeof(buf), "%Z", &t);
std::cout << "Current timezone: " << buf << std::end;
...
}

C++ convert date formats

I would like to convert an int date like:
20111201
to string:
01DEC2011
Is there a fast date format conversion built into C++ (or maybe a bash system command I can execute instead) to do this or am I stuck making a switch for all of the months?
You could use the strptime to convert your string to a struct tm, then use strftime to reformat it:
#include <ctime>
#include <iostream>
#include <sstream>
int main()
{
std::ostringstream date1;
date1 << 20111201;
struct tm tm;
strptime(date1.str().c_str(), "%Y%m%d", &tm);
char date2[10];
strftime(date2, sizeof(date2), "%d%b%Y", &tm);
std::cout << date1.str() << " -> " << date2 << std::endl;
}
Output is:
20111201 -> 01Dec2011
Just need to convert the Dec to upper case if it's necessary.
Don't use bash here. The way to go is to use Boost in C++ for more reasons than I've time to list here, but ultimately it will be just as fast as most other solutions you'll encounter and unless your functionality is absolutely time critical, it won't make a great deal of difference anyway.
Also, It's going to be far more flexible and maintainable than all those crappy little hard coded date conversion routines that you always encounter.
The following code will do what you want.
#include <iostream>
#include <sstream>
#include <boost/date_time/gregorian/gregorian.hpp>
#include <boost/algorithm/string.hpp>
using namespace boost::gregorian;
using namespace std;
int main(int argc, char **argv)
{
int dateIn = 20111201;
// Read the date in from ISO format as an int.
ostringstream ss;
ss << dateIn;
date d(from_undelimited_string( ss.str() ));
// Set the output format
date_facet *fct = new date_facet("%d%b%Y"); // [1]
locale loc = locale(locale::classic(), fct);
// Render the date as a string;
ss.str("");
ss.imbue(loc);
ss << d;
string dateOut( ss.str() );
boost::to_upper( dateOut );
cout << dateOut << endl;
}
This gives the following output:
01DEC2011
Just changing the format string "%d%b%Y" at ref [1] will change to a different output format but remember I've converted it to uppercase as well.
There's nothing directly built-in, since this format for dates
is relatively rare. The simplest solution here would be to
break the date up into year month day using % and /
operators (e.g. month is value / 100 % 100), then format the
three values normally, using std::ostream, and looking up the
date in a table. (This would obviously require some error
checking, since not all integral values yield valid dates.)
New answer to old question. This answer traffics through the C++11/14 <chrono> library instead of C's tm or boost::date_time. Otherwise it is very similar to the existing answers. It requires this free, open-source library for the parsing and formatting.
#include "tz.h"
#include <iostream>
#include <locale>
#include <sstream>
int
main()
{
auto date1 = 20111201;
std::stringstream stream;
stream.exceptions(std::ios::failbit);
stream << date1;
std::chrono::system_clock::time_point tp;
date::parse(stream, "%Y%m%d", tp);
auto str = date::format("%d%b%Y", tp);
auto& ct = std::use_facet<std::ctype<char>>(std::locale::classic());
ct.toupper(&str.front(), &str.back()+1);
std::cout << str << '\n';
}
I've included stream.exceptions(std::ios::failbit); to noisily detect invalid "integer dates". And I've included old C++98 code to convert the string to uppercase (the locale dance at the end).
01DEC2011
One of the advantages of using a modern C++ date/time library is the ease with which changes can be made. For example, what if now you need to parse the timestamp not with day-precision, but with millisecond precision? Here is how that might be done:
auto date1 = 20111201093357.275L;
std::stringstream stream;
stream.exceptions(std::ios::failbit);
stream << std::fixed << date1;
std::chrono::system_clock::time_point tp;
date::parse(stream, "%Y%m%d%H%M%S", tp);
auto str = date::format("%d%b%Y %T", tp);
auto& ct = std::use_facet<std::ctype<char>>(std::locale::classic());
ct.toupper(&str.front(), &str.back()+1);
std::cout << str << '\n';
which outputs:
01DEC2011 09:33:57.275000
Or perhaps these timestamps are known to originate from Chatham Island off the coast of New Zealand and you need them in UTC. Just add one line after the parse:
tp = date::locate_zone("Pacific/Chatham")->to_sys(tp);
And now the output is:
30NOV2011 19:48:57.275000
Taking into account arbitrary timezones and subsecond precision is currently beyond the capabilities of all other C++ libraries.

converting a timestring to a duration

At the moment I am trying to read in a timestring formatted and create a duration from that. I am currently trying to use the boost date_time time_duration class to read and store the value.
boost date_time provides a method time_duration duration_from_string(std::string) that allows a time_duration to be created from a time string and it accepts strings formatted appropriately ("[-]h[h][:mm][:ss][.fff]".).
Now this method works fine if you use a correctly formatted time string. However if you submit something invalid like "ham_sandwich" or "100" then you will instead be returned a time_duration that is not valid. Specifically if you try to pass it to a standard output stream then an assertion will occur.
My question is: Does anyone know how to test the validity of the boost time_duration? and failing that can you suggest another method of reading a timestring and getting a duration from it?
Note: I have tried the obvious testing methods that time_duration provides; is_not_a_date_time(), is_special() etc and they don't pick up that there is an issue.
Using boost 1.38.0
From the documentation, it looks like you may want to try using the stream operators (operator<<, operator>>); error conditions are described at Date Time Input/Output.
Alternately, I suppose you could validate the string before passing it in. Right offhand, it doesn't look like that particular method has any error handling.
Edit:
I'm not sure I would have thought to check the return value like this if it weren't for Brian's answer, but for completeness here's a full example that takes a string as input. You can either check the return value or have it throw an exception (I believe you'd want to catch std::ios_base_failure):
#include <iostream>
#include <sstream>
#include <string>
#include <boost/date_time/posix_time/posix_time.hpp>
using namespace std;
using namespace boost::posix_time;
int main(int argc, char **argv) {
if (argc < 2) {
cout << "Usage: " << argv[0] << " TIME_DURATION" << endl;
return 2;
}
// No exception
stringstream ss_noexcept(argv[1]);
time_duration td1;
if (ss_noexcept >> td1) {
cout << "Valid time duration: " << td1 << endl;
} else {
cout << "Invalid time duration." << endl;
}
// Throws exception
stringstream ss2;
time_duration td2;
ss2.exceptions(ios_base::failbit);
ss2.str(argv[1]);
try {
ss2 >> td2;
cout << "Time duration: " << td2 << endl;
} catch (ios_base::failure e) {
cout << "Invalid time duration (exception caught). what():\n"
<< e.what() << endl;
}
}
Use the stream operators.
time_duration td;
if (std::cin >> td)
{
// it's valid
}
else
{
// it isn't valid
}