Is this a conforming implementation of duration_cast?

Question

Sadly VS2012's duration_cast is broken, and I actually need the functionality which is broken. So, I wrote my own:

template<typename ToUnit, typename Rep, typename Period>
ToUnit duration_cast_2(const std::chrono::duration<Rep, Period>& right)
{
 typedef std::ratio_divide<Period, typename ToUnit::period>::type ratio;
 typedef std::common_type<std::common_type<typename ToUnit::rep, Rep>::type, intmax_t>::type common_type;
 return ToUnit(static_cast<typename ToUnit::rep>(static_cast<common_type>(right.count()) * static_cast<common_type>(ratio::num) / static_cast<common_type>(ratio::den)));
}

But I'm not entirely confident that it's correct.

• Loki Astari: Links don;t work for me. What is a duration_cast<> supposed to do?
• David: @LokiAstari Turn a std::chrono::duration<long long, std::micro> into a std::chrono::duration<double, std::milli>, for example. That conversion cannot be done implicitly, and needs the cast.

Answer 1

I can't say for sure about the internal logic but I can point some problems or things that could be improved in your piece of code:

• First of all, typename is missing before the names, making your code non-working with some compilers. I would also use using instead of typedef, but that's a mere matter of taste. Updated version:

  using ratio = typename std::ratio_divide<Period, typename ToUnit::period>::type;
  using common_type = std::common_type<typename std::common_type<typename ToUnit::rep, Rep>::type, intmax_t>::type;
  

• std::common_type is variadic. You don't have to bother with nested std::common_types:

  using common_type = typename std::common_type<typename ToUnit::rep, Rep, intmax_t>::type;
  

• I know that MSVC has not a full support for constexpr, but duration_cast is supposed to be constexpr (it should work with the November 2013 CTP though):

  template<typename ToUnit, typename Rep, typename Period>
  constexpr ToUnit duration_cast_2(const std::chrono::duration<Rep, Period>& right)
  {
   // ...
  }
  

• David: This was actually more than a year ago, and at the time I was using VS2010, so limited to not doing anything you mentioned above. I was most interested in the correctness of the internal logic. The whole thing was actually because there was a bug in the VS2010 duration_cast implementation. I don't know if it's fixed yet, probably is.
• Morwenn: @Dave From the link in your question, the problem seems to have been solved :)
• David: Oh, lol. Thanks. Also I guess I was using 2012 not 2010. I didn't reread my question, it was a long time ago =P