C How do I convert a std chrono time point to long and back

Question

I need to convert std  chrono  time point to and from a long type  integer 64 bits   I  m starting working with std  chrono      Here is my code   int main           std  chrono  time point lt std  chrono  system clock gt  now   std  chrono  system clock  now         auto epoch   now time since epoch        auto value   std  chrono  duration cast lt std  chrono  milliseconds gt  epoch       long duration   value count          std  chrono  duration lt long gt  dur duration        std  chrono  time point lt std  chrono  system clock gt  dt dur        if  dt    now          std  cout  lt  lt   Failure    lt  lt  std  endl      else         std  cout  lt  lt   Success    lt  lt  std  endl      This code compiles  but does not show success   Why is dt different than now at the end   What is missing on that code

User · Answer

time point objects only support arithmetic with other time point or duration objects   You ll need to convert your long to a duration of specified units  then your code should work correctly

User · Answer

std  chrono  time point lt std  chrono  system clock gt  now   std  chrono  system clock  now      This is a great place for auto   auto now   std  chrono  system clock  now      Since you want to traffic at millisecond precision  it would be good to go ahead and covert to it in the time point   auto now ms   std  chrono  time point cast lt std  chrono  milliseconds gt  now     now ms is a time point  based on system clock  but with the precision of milliseconds instead of whatever precision your system clock has   auto epoch   now ms time since epoch      epoch now has type std  chrono  milliseconds   And this next statement becomes essentially a no-op  simply makes a copy and does not make a conversion    auto value   std  chrono  duration cast lt std  chrono  milliseconds gt  epoch     Here   long duration   value count      In both your and my code  duration holds the number of milliseconds since the epoch of system clock   This   std  chrono  duration lt long gt  dur duration     Creates a duration represented with a long  and a precision of seconds   This effectively reinterpret casts the milliseconds held in value to seconds   It is a logic error   The correct code would look like   std  chrono  milliseconds dur duration     This line   std  chrono  time point lt std  chrono  system clock gt  dt dur     creates a time point based on system clock  with the capability of holding a precision to the system clock s native precision  typically finer than milliseconds    However the run-time value will correctly reflect that an integral number of milliseconds are held  assuming my correction on the type of dur    Even with the correction  this test will  nearly always  fail though   if  dt    now    Because dt holds an integral number of milliseconds  but now holds an integral number of ticks finer than a millisecond  e g  microseconds or nanoseconds    Thus only on the rare chance that system clock  now   returned an integral number of milliseconds would the test pass   But you can instead   if  dt    now ms    And you will now get your expected result reliably   Putting it all together   int main          auto now   std  chrono  system clock  now        auto now ms   std  chrono  time point cast lt std  chrono  milliseconds gt  now        auto value   now ms time since epoch        long duration   value count         std  chrono  milliseconds dur duration        std  chrono  time point lt std  chrono  system clock gt  dt dur        if  dt    now ms          std  cout  lt  lt   Failure    lt  lt  std  endl      else         std  cout  lt  lt   Success    lt  lt  std  endl      Personally I find all the std  chrono overly verbose and so I would code it as   int main          using namespace std  chrono      auto now   system clock  now        auto now ms   time point cast lt milliseconds gt  now        auto value   now ms time since epoch        long duration   value count         milliseconds dur duration        time point lt system clock gt  dt dur        if  dt    now ms          std  cout  lt  lt   Failure    lt  lt  std  endl      else         std  cout  lt  lt   Success    lt  lt  std  endl      Which will reliably output   Success    Finally  I recommend eliminating temporaries to reduce the code converting between time point and integral type to a minimum   These conversions are dangerous  and so the less code you write manipulating the bare integral type the better   int main          using namespace std  chrono         Get current time with precision of milliseconds     auto now   time point cast lt milliseconds gt  system clock  now            sys milliseconds is type time point lt system clock  milliseconds gt      using sys milliseconds   decltype now          Convert time point to signed integral type     auto integral duration   now time since epoch   count           Convert signed integral type to time point     sys milliseconds dt milliseconds integral duration           test     if  dt    now          std  cout  lt  lt   Failure    lt  lt  std  endl      else         std  cout  lt  lt   Success    lt  lt  std  endl      The main danger above is not interpreting integral duration as milliseconds on the way back to a time point   One possible way to mitigate that risk is to write       sys milliseconds dt sys milliseconds  duration integral duration      This reduces risk down to just making sure you use sys milliseconds on the way out  and in the two places on the way back in   And one more example   Let s say you want to convert to and from an integral which represents whatever duration system clock supports  microseconds  10th of microseconds or nanoseconds    Then you don t have to worry about specifying milliseconds as above   The code simplifies to   int main          using namespace std  chrono         Get current time with native precision     auto now   system clock  now           Convert time point to signed integral type     auto integral duration   now time since epoch   count           Convert signed integral type to time point     system clock  time point dt system clock  duration integral duration           test     if  dt    now          std  cout  lt  lt   Failure    lt  lt  std  endl      else         std  cout  lt  lt   Success    lt  lt  std  endl      This works  but if you run half the conversion  out to integral  on one platform and the other half  in from integral  on another platform  you run the risk that system clock  duration will have different precisions for the two conversions

User · Answer

as a single line   long value ms   std  chrono  duration cast lt std  chrono  milliseconds gt  std  chrono  time point cast lt std  chrono  milliseconds gt  std  chrono  high resolution clock  now    time since epoch    count

User · Answer

I would also note there are two ways to get the number of ms in the time point  I m not sure which one is better  I ve benchmarked them and they both have the same performance  so I guess it s a matter of preference  Perhaps Howard could chime in   auto now   system clock  now       Cast the time point to ms  then get its duration  then get the duration s count  auto ms   time point cast lt milliseconds gt  now  time since epoch   count       Get the time point s duration  then cast to ms  then get its count  auto ms   duration cast lt milliseconds gt  tpBid time since epoch    count      The first one reads more clearly in my mind going from left to right

[c++] C++ How do I convert a std::chrono::time_point to long and back

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