Difference between two time_point instances is not a duration?
c++, c++-chrono, g++
Solution
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>(...)`).
Problem
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".