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2054. time_point constructors need to be constexpr

Section: 29.6 [time.point] Status: Resolved Submitter: Anthony Williams Opened: 2011-05-13 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

In 29.6 [time.point], time_point::min() and time_point::max() are listed as constexpr. However, time_point has no constexpr constructors, so is not a literal type, and so these functions cannot be constexpr without adding a constexpr constructor for implementation purposes.

Proposed resolution: Add constexpr to the constructors of time_point. The effects of the constructor template basically imply that the member function time_since_epoch() is intended to be constexpr as well.

[2012, Portland]

Resolved by adopting paper n3469.

Proposed resolution:

This wording is relative to the FDIS.

1. Alter the class template definition in 29.6 [time.point] as follows:

   template <class Clock, class Duration = typename Clock::duration>
   class time_point {
     […]
   public:
     // 20.11.6.1, construct:
     constexpr time_point(); // has value epoch
     constexpr explicit time_point(const duration& d); // same as time_point() + d
     template <class Duration2>
       constexpr time_point(const time_point<clock, Duration2>& t);
   
     // 20.11.6.2, observer:
     constexpr duration time_since_epoch() const;
     […]
   };
   

2. Alter the declarations in 29.6.2 [time.point.cons]:

   constexpr time_point();
   

   -1- Effects: Constructs an object of type time_point, initializing d_ with duration::zero(). Such a time_point object represents the epoch.

   constexpr explicit time_point(const duration& d);
   

   -2- Effects: Constructs an object of type time_point, initializing d_ with d. Such a time_point object represents the epoch + d.

   template <class Duration2>
     constexpr time_point(const time_point<clock, Duration2>& t);
   

   -3- Remarks: This constructor shall not participate in overload resolution unless Duration2 is implicitly convertible to duration.

   -4- Effects: Constructs an object of type time_point, initializing d_ with t.time_since_epoch().

3. Alter the declaration in 29.6.3 [time.point.observer]:

   constexpr duration time_since_epoch() const;
   

   -1- Returns: d_.