How to get std::chrono::system_clock::time_point from std::chrono::system_clock::time_point.time_since_epoch().count()?

Question

I have a variable stored as a long a value from std::chrono::system_clock::time_point.time_since_epoch().count() . I would now like to restore that std::chrono::system_clock::time_point from the long variable. So how can I convert a long to a std::chrono::system_clock::time_point ?

Answer 1

You first need to convert the integral type to a chrono::duration , and then convert the duration to a system_clock::time_point . But there's a catch:

duration is a template:

template <class Rep, class Period> class duration;

If you convert the integral type to the wrong duration , you'll get the wrong time_point .

Fortunately system_clock itself tells you the correct duration with its nested duration type: system_clock::duration . Additionally, each of these conversions is explicit .

So, in summary:

using namespace std::chrono;
system_clock::time_point tp{system_clock::duration{i}};