What precision is used by std::chrono::system

Here's some code that I found:

std::cout << std::chrono::system_clock::now().time_since_epoch().count() << std::endl;

This prints 1662563612364838407 for me; so it looks like this prints the number of nanoseconds since the UNIX epoch (1970-01-01).

But is this precision guaranteed? I didn't find any indication at https://en.cppreference.com/w/cpp/chrono/system_clock that this value will always be in nanoseconds. Is it possible that this will return e.g. microseconds with another compiler, operating system or hardware?

CodePudding user response：

No it is not guaranteed. You can use the clocks period member alias to get tick period in seconds:

#include <chrono>
#include <iostream>

int main() {
    std::cout << std::chrono::system_clock::period::num << " / " << std::chrono::system_clock::period::den;
}

Possible output:

1 / 1000000000

CodePudding user response：

Is it possible that this will return e.g. microseconds with another compiler, operating system or hardware?

Yes, but you can always std::chrono::duration_cast it into a known duration unit. If you want it in seconds for example:

auto dur = std::chrono::duration_cast<std::chrono::seconds>(
               std::chrono::system_clock::now().time_since_epoch());

std::cout << dur << '\n';

Possible output:

1662575635s

Pre C 20:

std::cout << dur.count() << '\n';

1662575635

Note: Stay within the chrono domain until it's absolutely necessary to leave it (using .count() etc).