Re: [boost] [gsoc 2013] draft proposal for chrono::date

Subject: Re: [boost] [gsoc 2013] draft proposal for chrono::date
From: Howard Hinnant (howard.hinnant_at_[hidden])
Date: 2013-05-05 23:29:50

On May 5, 2013, at 1:37 PM, Howard Hinnant <howard.hinnant_at_[hidden]> wrote:

> On May 5, 2013, at 1:09 PM, Anders Dalvander <boost_at_[hidden]> wrote:
> 
>> Sorry about that, should be:
>> 
>> chrono::date d = year(2013) / may / day(5);
>> chrono::time t = hours(12) + minutes(34) + seconds(56);
>> chrono::date_time dt(d, t);
>> // Here `dt` should be the representation of May 5th 2013 at 12:34:56, but that in turn occurred at different instants for different people around the world.
>> // In order to convert it to a `chono::system_clock::time_point` a timezone would be needed:
>> chono::system_clock::time_point instant = dt.in_timezone(chrono::timezones::utc); // or perhaps `dt.in_utc()` for short.
> 
> Fwiw, I just ran this program fragment:
> 
>    day_point d = year(2013) / may / day(5);
>    auto t = hours(13) + minutes(34) + seconds(30);
>    auto dt = d + t;
>    auto EDT = hours(-4);
>    dt -= EDT;
>    auto dt_now = system_clock::now();
>    auto diff = dt_now - dt;
>    std::cout << duration_cast<seconds>(diff).count() << " seconds\n";
> 
> And it output:
> 
> 1 seconds

Btw, I'm getting better at this (speaking of manual dexterity more than coding):

    day_point d = year(2013) / may / day(5);
    auto t = hours(22) + minutes(55) + seconds(55);
    auto dt = d + t;
    auto EDT = -hours(4);
    dt -= EDT;
    auto dt_now = system_clock::now();
    auto diff = dt_now - dt;
    std::cout << duration_cast<milliseconds>(diff).count() << " ms\n";

Output:

318 ms

-------------------------

My point in this post is that we have an existing std::chrono date_time facility and it would be good to interoperate with it.  Dissecting this code line by line:

    day_point d = year(2013) / may / day(5);

d is a std::chrono::time_point with the same epoch as std::chrono::system_clock::time_point, but has a much coarser resolution (technically day_point::period):  std::ratio<86400>.

Because d has the same epoch as system_clock::time_point, the two types can, by C++11 rules, have arithmetic together.  You can subtract one from the other and it just works.  The result is a std::chrono::duration with the precision of the common_type of the two precisions (typically the finer).

So, the above code is proposed.  But much of the following code is simply existing C++11, showing interoperability with the proposed code with C++11.

    auto t = hours(22) + minutes(55) + seconds(55);

Nothing new about the above code.  This is just a std::chrono::duration with a resolution of seconds.  It exists today.

    auto dt = d + t;

The proposal is that d has type time_point<system_clock, duration<int, ratio<86400>>>.  If that is so, then it is, by C++11 rules, already legal to add such a time_point to a duration of seconds.  The result is a

    time_point<system_clock, duration<system_clock::rep, ratio<1>>>

I.e. the result is a time_point with the same epoch as system_clock.  A rep capable of holding the union of values from both d and t, and a precision capable of holding both all values from d and all values from t.  In English: dt is a count of seconds from the system_clock epoch ... all by C++11 specification.  Nothing invented here except day_point in the first line.

    auto EDT = -hours(4);

This is just C++11.  A duration type.  It could have any units: hours, minutes, seconds, whatever.

    dt -= EDT;

As dt is a timePoint with precision seconds, as long as the UTC offset does not have finer precision than seconds, this just works according to existing C++11 time_point arithmetic.

    auto dt_now = system_clock::now();

The above line is straight out of C++11.  Nothing new here.

    auto diff = dt_now - dt;

Since dt_now and dt are both time_points based on system_clock, this is also straight out of C++11.  Arithmetic between the two time_points is well defined by C++11, and results in a duration with a precision of the finer of the two time_points (typically that of system_clock::time_point, which for me has a precision of microseconds).

    std::cout << duration_cast<milliseconds>(diff).count() << " ms\n";

For me this is just a simple C++11 cast of microseconds to milliseconds.

-------------------------

In summary:  If the serial date type is nothing more than a time_point<system_clock, duration<int, ratio<86400>>>, then we get a great deal of interoperability with the C++11 time_point/duration facilities for free.  And with absolutely no loss in performance. We get at no cost serial date types and serial date_time types.  And the only invention needed is the field types, and the interaction between the invented field types, and the already standardized serial types.

Howard