Subject: Re: [boost] [gsoc 2013] draft proposal for chrono::date
From: Anders Dalvander (boost_at_[hidden])
Date: 2013-05-07 17:32:44
On 2013-05-06 05:29, Howard Hinnant wrote:
>
> 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;
The above statement is the one which troubles me. Before executing the
statement `dt` is supposedly in EDT, and after the statement has
executed `dt` is in UTC (or is it the other way around). Anyway `dt` is
of type `time_point<system_clock, duration<system_clock::rep,
ratio<1>>>` as you wrote, but that type has a defined meaning. Each
`time_point` is a single point in time, an instant. If we start to make
exceptions to this rule, then we're back with the problems of
`boost::date_time::ptime`.
And what about the `system_clock::to_time_t` and
`time_point::time_since_epoch` functions? Shouldn't it be possible to
call them at any given time and not worry if someone has adjusted the
time_point to be in the "correct" timezone?
This is one of the things that I hoped the chrono library would solve by
requiring the user to explicit.
The C-library is this explicit, why shouldn't the chrono library be able
to be explicit as well?
> 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.
Yes, we get a great deal of interoperability for free, for sure, but I
worry that all the implicit casting will be an even larger debt.
Regards,
Anders Dalvander