Detecting DST changes in C++ - c++

Question

I am working on c++ real time application which is doing lot of date manipulations. For performance reasons, I made the UTC offset as configurable value which is read only once at the time of application initialization. But it is causing issues in DST zones.
When DST Changes happens, My UTC offset variable contain wrong value.
Calculating the Offset every time is not a optimal solution for me.
So is there anyway to notify my application about DST changes? So I can calculate the offset only when it need to modified.

Answer 1

Using the draft C++20 <chrono> facilities you can discover the std::chrono::system_clock::time_point for the next UTC offset change for any time zone, and then std::this_thread::sleep_until that time_point. When you wake, do whatever it is you want to do, such as compute a new UTC offset.
The new draft C++20 <chrono> library has been prototyped and is available as a free, open-source library under namespace date. This library is portable across VS, gcc and clang, and operates with C++11 and later.
Here is a sketch of how this might be coded:
#include "date/tz.h"
#include <thread>
template <class F>
void
on_utc_offset_change(date::time_zone const* tz, F f)
{
using namespace date;
using namespace std;
using namespace std::chrono;
while (true)
{
auto info = tz->get_info(system_clock::now());
f(info);
this_thread::sleep_until(info.end);
}
}
A time_zone has a get_info(system_clock::time_point) member function that returns a sys_info. The sys_info contains all information about a time_zone for a particular point in time:
struct sys_info
{
sys_seconds begin;
sys_seconds end;
std::chrono::seconds offset;
std::chrono::minutes save;
std::string abbrev;
};
The begin and end members are seconds-precision system_clock time_points that delineate the range [begin, end) for which this time_zone has this UTC offset and abbreviation (abbrev).
This function might be called like this:
auto lambda = [](date::sys_info const& info)
{
using namespace date;
std::cerr << "Current UTC offset: " << info.offset << '\n';
std::cerr << "Current tz abbreviation: " << info.abbrev << '\n';
std::cerr << "Sleeping until " << info.end << " UTC\n";
};
std::thread{on_utc_offset_change<decltype(lambda)>,
date::current_zone(),
lambda}.detach();
This detaches a thread that runs forever and every time the UTC offset changes for the computer's current time_zone (at the time the thread was launched), it prints out current information, for example.
Current UTC offset: -14400s
Current tz abbreviation: EDT
Sleeping until 2018-11-04 06:00:00 UTC
Alternatively you could run this for a specific time_zone which is not the computer's currently set time_zone with:
std::thread{on_utc_offset_change<decltype(lambda)>,
date::locate_zone("America/New_York"),
lambda}.detach();