[Interest] QDateTime and std::chrono

[Giuseppe D'Angelo]

Hi,

Il 02/11/18 16:40, Jérôme Godbout ha scritto:
> Maybe you can pass by a string, this would be highly inefficient but could be simple enough. I guess you should make the time into UTC too. You could use QString to std::string for the string stream. And do the following:
> 
> std::tm tm = {};
> std::stringstream ss("Jan 9 2014 12:35:34"); // Change this for the QDateTime toString().toStdString()
> ss >> std::get_time(&tm, "%b %d %Y %H:%M:%S");
> auto tp = std::chrono::system_clock::from_time_t(std::mktime(&tm));
> 
> I don't think it's the best solution, but could work easily if performance is not an issue. Hope someone have a better plan...

This is a tad overkill, you can use QDateTime::toMSecsSinceEpoch(), 
divide by 1000 and build a std::time_t out of it. (Remember to check for 
overflows, as you don't know what type std::time_t actually is.)

But also, why going down this route? Can't you simply build a 
std::chrono::system_clock::time_point out of a duration? I.e. 
(pseudocode, not tested):

> QDateTime dt;
> std::chrono::milliseconds duration{dt.toMSecsSinceEpoch()};
> std::chrono::system_clock::time_point tp{duration};

Last, but not least, note that std::system_clock is a Unix clock only 
starting in C++2a; before you had no guarantees. Does anyone know if 
QDateTime::toMSecsSinceEpoch() returns UTC time or Unix time?


Cheers,
-- 
Giuseppe D'Angelo | giuseppe.dangelo at kdab.com | Senior Software Engineer
KDAB (France) S.A.S., a KDAB Group company
Tel. France +33 (0)4 90 84 08 53, http://www.kdab.com
KDAB - The Qt, C++ and OpenGL Experts

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