Difference between two time_point instances is not a duration?

Question

I don't understand why this code chokes with g++ 4.7.2:

#include <chrono>

main ()
{
    std::chrono::system_clock::time_point t1, t2 ;
    std::chrono::seconds delay ;

    t1 = std::chrono::system_clock::time_point::max () ;
    t2 = std::chrono::system_clock::now () ;
    delay = t1 - t2 ;
    // t1 = t2 + delay ;
    // t1 = t2 - delay ;
}

with the error:

test.cc: In function ‘int main()’:
test.cc:10:18: error: no match for ‘operator=’ in ‘delay = std::chrono::operator,<std::chrono::system_clock, std::chrono::duration<long int, std::ratio<1l, 1000000l> >, std::chrono::duration<long int, std::ratio<1l, 1000000l> > >((*(const std::chrono::time_point<std::chrono::system_clock, std::chrono::duration<long int, std::ratio<1l, 1000000l> > >*)(& t1)), (*(const std::chrono::time_point<std::chrono::system_clock, std::chrono::duration<long int, std::ratio<1l, 1000000l> > >*)(& t2)))’

It seemed to me that "time_point - time_point" gives a "duration".

Answer 1

It does produce a duration, but there are different kinds of durations. std::chrono::duration is templatized on a representation type and a unit ratio. std::chrono::seconds for example has a unit ratio of 1, while std::chono::nanoseconds has a unit ratio of std::nano, or 1/1000000000. time points have the same template parameters.

The specific unit ratio of std::chrono::system_clock::time_point is implementation defined, but it is almost certainly less than than that of std::chrono::seconds. As such, the duration produced from subtracting those two time points has much more precision than can be represented by std::chrono::seconds. The default behaviour is to not allow assignments that lose precision with durations that have integer representations. So you can either use a duration with enough precision (std::chrono::system_clock::duration) or cast the result to the duration you want (std::chrono::duration_cast<std::chrono::seconds>(...)).

Answer 2

time_point - time_point does return a duration, just not the one in the code. You could replace std::chrono::seconds with std::chrono::system_clock::duration, or you could use a duration_cast to convert to the kind you need.

Answer 3

The difference between two time points is indeed a duration; but you can't implicitly convert one duration type to another, since that could silently lose precision.

If you want to reduce the precision from system_clock::duration to seconds, then you need to make the conversion explicit using a duration_cast:

delay = duration_cast<std::chrono::seconds>(t1 - t2);

Alternatively, you might want to retain the precision of the system clock:

auto delay = t1 - t2; // Probably microseconds, or nanoseconds, or something